ES3035896T3 - Light shuttering for waveguide pupil expansion - Google Patents

Abstract

A light-shuttering device comprises several liquid crystal cells, each operating in a first or second optical state in response to a first or second trigger signal. A trigger circuit comprises several switches and a controller. Each switch emits the first or second trigger signal to the corresponding liquid crystal cell. The controller sequentially updates the output of each switch during a refresh cycle. The trigger circuit determines the refresh order of the switches during a refresh cycle, based on changes in the trigger signals during the refresh.

Description

DESCRIPTION

Light shutter for waveguide pupil expansion

Field

The present disclosure relates to image projection. More specifically, the present disclosure relates to holographic projection. Some embodiments relate to virtual image projection. Other embodiments relate to real image projection. Some embodiments relate to viewing an image projected through a waveguide. Some embodiments relate to a control device, such as a light shutter device, and a method for controlling the light of an image projected through a waveguide. Some embodiments relate to a display system comprising an image generating unit and a waveguide arranged with a light shutter device. Some embodiments relate to a head-up display (HUD).

Background and introduction

The light scattered by an object contains amplitude and phase information. This phase and amplitude information can be captured, for example, on a photosensitive plate using known interference techniques to form a holographic recording, or "hologram," comprising interference fringes. The hologram can be reconstructed by illumination with appropriate light to form a two- or three-dimensional holographic reconstruction, or reproduction image, representative of the original object.

Computer-generated holography can numerically simulate the interference process. A computer-generated hologram can be calculated using a technique based on a mathematical transformation, such as the Fresnel transform or the Fourier transform. These types of holograms may be referred to as Fresnel/Fourier transform holograms or simply Fresnel/Fourier holograms. A Fourier hologram can be considered a planar/Fourier-domain representation of the object or a planar/frequency-domain representation. A computer-generated hologram can also be calculated using coherent ray tracing or a point cloud technique, for example.

A computer-generated hologram can be encoded in a spatial light modulator arranged to modulate the amplitude and/or phase of the incident light. Light modulation can be achieved using electrically addressable liquid crystals, optically addressable liquid crystals, or micromirrors, for example.

A spatial light modulator typically comprises a plurality of individually addressable pixels, also referred to as cells or elements. The light modulation scheme can be binary, multilevel, or continuous. Alternatively, the device can be continuous (i.e., not composed of pixels), and thus the light modulation can be continuous throughout the device. The spatial light modulator can be reflective, meaning that the modulated light exits by reflection. The spatial light modulator can also be transmissive, meaning that the modulated light exits by transmission.

A holographic projector can be provided using the system described herein. These projectors have been used, for example, in head-up displays (HUDs) and light detection and ranging (LIDAR) systems.

EP0136085A1 discloses an electro-optical device for influencing the presentation of visual information when placed before the eyes of an observer, comprising one or more cholesteric liquid crystal cells which, under the control of an electrical drive signal, can be rapidly switched, by means of the cholesteric-nematic phase change effect, between a clear transparent texture and a turbid, scattered texture.

US2018188528A1 discloses an optical device including a waveguide including an internal coupling optical element configured to couple light into the waveguide, a light distributing element configured to receive light from the internal coupling optical element and distribute light at a selected wavelength, and an external coupling optical element configured to receive light from the light distributing element and couple light out of the waveguide.

Compendium

Aspects of the present disclosure are defined in the appended independent claims.

The present disclosure relates to image projection. It relates to an image projection method and an image projector comprising a display device. The present disclosure also relates to a projection system comprising the image projector and a viewing system.

The present disclosure is particularly applicable to a binocular vision system. The vision system may comprise an eye or eyes of the observer. The vision system comprises an optical element with optical power (e.g., one or more lenses of the human eye) and a viewing plane (e.g., the retina of the human eye). The image projector may be referred to as a "display system" or "light engine." The display device and the image formed (or perceived) thereby are spatially separated from one another. The image is formed, or perceived by the observer, in a viewing plane. In some embodiments, the image is a virtual image and the viewing plane may be referred to as a virtual image plane. The image is formed by illuminating a diffractive pattern of a diffractive structure (e.g., a hologram) displayed on a display device, such as a spatial light modulator (e.g., a liquid crystal on silicon LCOS spatial light modulator).

Our co-pending UK patent application GB2108456.1, filed on June 14, 2021, entitled "Waveguide Pupil Expansion," discloses examples of such a projection system.

The projection system uses a waveguide in the optical path between the image projector and the viewing system. This waveguide expands the field of view and thus the maximum propagation distance, accessible with the full diffraction angle of the display device. Furthermore, the use of a waveguide can laterally enlarge the user's eyecup, allowing some eye movement without affecting the image. Therefore, the waveguide may be referred to as a pupil expander.

In the projection system disclosed in GB2108456.1, the display device displays a special type of hologram that is configured to direct light into a plurality of channels, referred to as "holographic channels," where each channel corresponds to a different portion (i.e., a subarea) of the image. Each holographic channel is emitted from multiple different transmission points in the waveguide, toward the viewing system, such that light from the same holographic channel propagates from each transmission point at the same angle or range of angles. Therefore, each holographic channel may be referred to as an "angular channel." In other words, each holographic channel may comprise light spatially modulated according to a different portion of the image and thus corresponds to distinct image content. Light of the same image content may be emitted from multiple different transmission points in the waveguide, all at the same angle or within the same angular range with respect to a waveguide surface, even though the transmission points are spatially separated from each other along the exit surface that forms the "exit hole" of the waveguide.

The projection system may be arranged so that no more than one instance of each holographic channel reaches a single eye, aperture, or viewing window of the viewing system. However, it is common for a viewing system to have multiple apertures, each with a different location and, therefore, a distinct viewing position. For example, a human observer typically views an image within a viewing window or "eye box" with two spatially separated eyes. Therefore, there is a risk that multiple instances of light from the same holographic channel (i.e., a common holographic channel) reach multiple apertures or viewing windows (also referred to as "entrance pupils") of the viewing system, virtually simultaneously. If this were to happen, the observer's brain—or the processor associated with a machine vision system—would perceive that both eyes (or both, or each of the viewing apertures) had received light from the same image content (i.e., light from the same part or point of the image) from the same angle, even though those eyes or viewing apertures are at different viewpoints. This is counterintuitive for an observer or a vision system, since, according to well-established mathematical principles, two different viewpoints should, normally, receive light from a common point at different angles.

Accordingly, GB2108456.1 proposes a control device and a method for controlling light emitted by a waveguide and addressing this problem. Embodiments comprise a control device in the form of a "light shutter device," which can be thought of as a "waveguide aperture." The light shutter device may be configured to selectively block or impede the propagation of spatially modulated light emitted by the waveguide and to allow the propagation of the remainder of said light toward the vision system. In particular, the light shutter device may selectively block or impede the propagation of one or more channels of spatially modulated light emitted by the waveguide and to allow the propagation of one or more respective channels toward the vision system. The control device may comprise one or more holes, or apertures, and one or more walls or barriers for selectively emitting and blocking light from the waveguide. The control device may be dynamically configured to select and/or change which portion(s) of the device act(s) as the opening and which other portion(s) act(s) as the barrier at a given time. The control device may comprise a plurality of "configurations" or "light-blocking configurations" that define the arrangement, including the size and position, of the openings and barriers. The light-blocking configurations of the control device may be predetermined or determined in real time. The light-blocking configurations are also referred to herein as "stages" or "phases" of the control device.

The present disclosure relates to improvements in the control device. In particular, the described control device comprises a light-shuttering device positioned adjacent to the external exit surface (i.e., the exit hole) of the waveguide, which forms the expanded exit pupil. The light-shuttering device is therefore positioned along the optical path between the waveguide and the vision system. The improved control device may have a particular application for selectively blocking light, such that light from the same holographic channel, transmitted from different points in the waveguide, does not reach more than one of the entrance pupils of the vision system, at substantially the same time.

As described in GB2108456.1, it is desirable for the control device to be arranged so that the light-blocking setting can be changed dynamically or switched rapidly. In particular, in some implementations, it is desirable for each switchable element of the control device to be able to update in a time period of less than 0.1 ms.

The light shutter device of the present disclosure is configured as a plurality of pixels, as a one-dimensional array, wherein each pixel forms a "shutter" to selectively block light from a corresponding transmission point along the waveguide. In some embodiments, the pixels are liquid crystal cells, operable in two optical states. In one optical state, the liquid crystal cell blocks (e.g., absorbs or reflects) incident light from a corresponding position (or region) in the waveguide, to prevent light transmission. In the other optical state, the liquid crystal cell is transparent, such that incident light from a corresponding position (or region) in the waveguide is transmitted therethrough. The pixels of the light shutter device are controlled by control signals, as is well known in the art. However, for some liquid crystal cells, such as the pixels in a thin-cell liquid crystal device, the time to switch from a second optical state to a first optical state is longer than the time to switch from the first optical state to the second optical state, or vice versa. For example, the switching time is longer when the liquid crystal cell "relaxes" to its new state (the first optical state) compared to when it is "driven" to its new state (the second optical state).

The present disclosure addresses the technical problem that the switching time period of the pixels of a light shutter device, when switching in a direction in which the liquid crystal cell "relaxes" to its new state, is too long for the application requirements, thereby compromising performance. In particular, if the light shutter device does not switch quickly enough—such as in the time period associated with the light source turning off or "gating" during a display frame refresh—some pixels in the device may not be updated when the next image frame is displayed. Importantly, the configuration update of the light shutter device may be incomplete. Consequently, there is a risk that light may not be blocked or may be prevented from propagating from the waveguide during a given frame. For example, light that should be blocked may propagate to the vision system, and/or light that should be allowed to propagate to the vision system may be blocked. In some examples, there is a risk that light from the same angular holographic channel may still propagate from multiple transmission points from the waveguide to the vision system, as described above.

According to the claimed invention, each pixel of the light shutter device is directly driven by one of the following signals: a first drive signal and a second drive signal, wherein the first drive signal switches the pixel to the first optical state and the second to the second. Furthermore, during a refresh cycle that updates the configuration of the light shutter device, the order in which the pixels are driven is determined based on changes in the respective drive signals that occur during the refresh. In particular, the order in which the pixels of the light shutter device are driven may be determined to compensate for the switching time period of the pixel. In particular, the order in which the pixels of the light shutter device are driven is determined such that any relatively slow state changes occur first. In this manner, the light shutter device may advantageously be updated quickly (in particular, quickly enough to at least match the frame rate of the display device with which it is used). This significantly reduces the risk of blocking or incorrectly transmitting parts or light channels during a given frame due to an incomplete pixel refresh cycle.

Accordingly, a drive circuit is provided for driving a light shutter device comprising a plurality of liquid crystal cells. Each liquid crystal cell operates in a first or second optical state in response to a first or second drive signal. The drive circuit comprises a plurality of switches. Each switch is arranged to output the first or second drive signal to the respective liquid crystal cell. The drive circuit further comprises a drive controller arranged to sequentially update the output of each switch during a refresh cycle. The drive circuit is arranged to determine the sequential refresh order of the digital switches during a refresh cycle, based on changes in the drive signals during the refresh.

The first optical state of the liquid crystal cell may be transparent or "off," and the second may be opaque or "on." In accordance with the claimed invention, liquid crystal cells take longer to change from the second optical state to the first than from the first to the second. In particular, liquid crystal cells typically take longer to change when "relaxing" to a new state, such as when changing from the second optical state to the first (e.g., when going from opaque to transparent or from "on" to "off"). In accordance with the claimed invention, the driving circuit is arranged during a refresh cycle to update the output of any switch that switches from the second drive signal to the first before any other digital switch. Therefore, liquid crystal cells that take the longest to reach their new optical state are switched first during the sequential refresh, ensuring that all liquid crystal cells are fully refreshed at the end of the refresh cycle. In other words, the drive circuit is arranged to compensate for the slower switching time (relaxation time) from the second optical state to the first optical state.

In some embodiments, the plurality of switches comprises a plurality of digital switches. Each digital switch comprises a first input arranged to receive the first drive signal and a second input configured to receive the second drive signal. In some embodiments, a capacitor is associated with each digital switch. Each capacitor provides a control input to the corresponding digital switch in response to a signal from the drive controller during a refresh cycle, to selectively output either the first or the second drive signal to the corresponding liquid crystal cell. In this manner, the capacitor maintains the control input to the digital switch between refreshes such that the drive signal output from the digital switch to the respective liquid crystal cell remains stable.

In some embodiments, the trigger circuit further comprises a multiplexing circuit for sequentially routing control signals from the trigger controller to update the output of each of the switches during an update cycle.

In some embodiments, the liquid crystal cells of the light-shuttering device are arranged in a one-dimensional array. In some examples, the light-shuttering device is arranged to restrict the area of the output face of an optical replicator (e.g., a waveguide) visible from a viewing plane. In some implementations, the optical replicator is arranged to generate a plurality of replicas of a hologram. The light-shuttering device is disposed between a display device and a vision system, such as a viewfinder.

In some embodiments, at least one (or, optionally, a plurality of) update cycles are completed within the integration time of the human eye. In some embodiments, an update cycle is initiated if a change in the observer's viewing position is detected.

Further provided is a light engine (or display system or image projector) arranged to form an image visible from a viewing window. The light engine comprises a display device, an optical replicator, and a light shutter device, as described herein. The display device is arranged to display a hologram of the image and spatially modulate the light in accordance with said hologram. The optical replicator comprises a waveguide pupil expander, arranged to receive the spatially modulated light and provide a plurality of different light propagation paths for the spatially modulated light from the display device to the viewing window. The light shutter device is disposed between the waveguide and the viewing window. In some embodiments, the hologram is configured to angularly distribute the spatially modulated light of the image according to the position of the image content, such that angular channels of the spatially modulated light correspond to respective continuous regions of the image.

In some embodiments, at least one liquid crystal cell of the plurality of cells of the light shutter device forms an aperture arranged such that a first viewing position within the viewing window receives spatially modulated light with a first associated light propagation path through the optical replicator, and a second viewing position within the viewing window receives spatially modulated light with a second associated light propagation path through the optical replicator. The first light propagation path is different from the second. In some embodiments, the spatially modulated light associated with the first light propagation path is a first spatially modulated light channel, and the spatially modulated light associated with the second light propagation path is a second spatially modulated light channel. In particular, at least one liquid crystal cell of the plurality of liquid crystal cells of the light shutter device may form an opening arranged such that a first viewing position within the viewing window receives a first light channel spatially modulated by the hologram according to a first image region and a second viewing position within the viewing window receives a second light channel spatially modulated by the hologram according to a second image region.

In some embodiments, the first and second regions are adjacent regions of the image. In some embodiments, adjacent angular channels of the spatially modulated light correspond to adjacent regions of the image. In some embodiments, the first and second regions of the image substantially do not overlap.

In some embodiments, the image is a virtual image perceived upstream of the display device. In some embodiments, the light shutter device is coupled to an output face of the waveguide pupil expander. In some embodiments, the light shutter device restricts the area of the waveguide output face visible from the viewing window. In some embodiments, the waveguide pupil expander and the viewing window are not parallel.

In some embodiments, the control device comprises at least one hole, e.g., a plurality of holes. Each hole may provide the first and/or second viewing positions with a respective light propagation path of spatially modulated light, such that spatially modulated light that has propagated via different light propagation paths is delivered, respectively, to the first and second viewing positions at substantially the same time. When the hologram is a channeling hologram, configured to angularly distribute the spatially modulated image light according to the position of the image content, the control device may comprise at least one hole, e.g., a plurality of holes, wherein each hole provides the first and/or second viewing positions with a channel of spatially modulated light, such that different image content is respectively distributed to the first and second viewing positions at substantially the same time. In some embodiments, the respective channels do not overlap, but are continuous.

In some embodiments, the control device is configured such that each hole is switchable between an open and a closed position, providing a plurality of different configurations, wherein each control device configuration comprises an alternating sequence of holes and closures. The control device may be configured to provide a first control device configuration at a first time and a second control device configuration at a second time, wherein the first control device configuration and the second control device configuration are complementary. Advantageously, the time interval between the first time and the second time is less than the integration time of the human eye.

In some embodiments, a first control device configuration provided by the control device delivers modulated light according to the first and third image regions to the first viewing position, and modulated light according to the second and fourth image regions of the image to the second viewing position, wherein the first through fourth regions are contiguous and ordered areas of the image. In some embodiments, a second control device configuration provided by the control device delivers modulated light according to the second and fourth image regions of the image to the first viewing position, and modulated light according to the first and third image regions of the image to the second viewing position. In some embodiments, the image content of any of the first through fourth image regions, as delivered at the first viewing position, is not identical to the image content of the corresponding region of the first through fourth image regions, as provided at the second viewing position.

In some embodiments, the first viewing position and the second viewing position are the first and second eye positions of an observer, and the viewing window is an eyebox.

In some embodiments, a size and/or location, within the control device, of at least one of the holes is dynamically variable.

The term "hologram" refers to a recording containing amplitude or phase information, or a combination of both, related to the object. The term "holographic reconstruction" refers to the optical reconstruction of the object formed by illuminating the hologram. The system described herein is described as a "holographic projector" because the holographic reconstruction can be a real image and spatially separate from the hologram. The term "reproduction field" refers to the 2D area within which the holographic reconstruction is formed and fully focused. If the hologram is displayed on a spatial light modulator comprising pixels, the reproduction field will be repeated in the form of a plurality of diffracted orders, where each diffracted order is a replica of the zero-order reproduction field. The zero-order reproduction field generally corresponds to the preferred or main reproduction field, since it is the brightest. Unless explicitly stated otherwise, the term "reproduction field" should be understood as referring to the zero-order reproduction field. The term "playback plane" refers to the spatial plane containing all the playback fields. The terms "image," "playback image," and "image region" refer to the areas of the playback field illuminated by the holographic reconstruction light. In some embodiments, the "image" may comprise discrete points, which may be referred to as "image points" or, for convenience, "image pixels."

The terms "encoding," "writing," or "addressing" are used to describe the process of providing the plurality of pixels in the SLM with a plurality of control values that determine the modulation level of each pixel. The pixels in the SLM can be said to be configured to "display" a distribution of light modulation upon receiving the plurality of control values. Therefore, the SLM can be said to "display" a hologram, and this can be considered a grouping of light modulation values or levels.

It has been discovered that a holographic reconstruction of acceptable quality can be generated from a "hologram" containing only phase information related to the original object (i.e., the target image for reconstruction). Such a holographic recording may be referred to as a phase-only hologram. Embodiments relate to a phase-only hologram, but the present disclosure is equally applicable to amplitude-only holography. The present disclosure is not limited to any particular method of calculating holograms. Some embodiments relate to point cloud holograms (i.e., holograms generated by point cloud methods), by way of example only. However, the present disclosure is equally applicable to Fourier- or Fresnel-type holograms and to holograms calculated by other techniques, such as coherent ray tracing.

The present disclosure is also applicable to forming a holographic reconstruction using amplitude and phase information related to the original object (i.e., the target image). In some embodiments, this is achieved by complex modulation using a fully complex hologram containing amplitude and phase information related to the original object. This hologram may be referred to as a fully complex hologram because the value (gray level) assigned to each pixel has amplitude and phase components. The value (gray level) assigned to each pixel may be represented as a complex number with amplitude and phase components. In some embodiments, a computer-generated fully complex hologram is calculated.

The phase value, phase component, phase information, or simply the phase of the pixels in the computer-generated hologram or spatial light modulator may be referred to as shorthand for "phase delay." That is, any phase value described is, in fact, a number (e.g., in the range of 0 to 2n) that represents the amount of phase delay provided by that pixel. For example, a pixel in the spatial light modulator described as having a phase value of n/2 will delay the phase of the received light by n/2 radians. In some embodiments, each pixel in the spatial light modulator may operate at one of a plurality of possible modulation values (e.g., phase delay values). The term "gray level" may be used to refer to the plurality of available modulation levels. For example, the term "gray level" may be used for convenience to refer to the plurality of phase levels available in a phase-only modulator, even though different phase levels do not provide different shades of gray. The term "gray level" may also be used for convenience to refer to the plurality of complex modulation levels available in a complex modulator.

Thus, the hologram comprises a grouping of gray levels, i.e., a grouping of light modulation values, such as a grouping of phase delay values or complex modulation values. The hologram is also considered a diffractive pattern because it causes diffraction when viewed in a spatial light modulator and illuminated with light with a wavelength comparable to, and generally shorter than, the inter-pixel pitch of the spatial light modulator. Reference is made herein to the combination of the hologram with other diffractive patterns, such as diffractive patterns functioning as a lens or a grating. For example, a diffractive pattern functioning as a grating may be combined with a hologram to translate the playback field into the playback plane, or a diffractive pattern functioning as a lens may be combined with a hologram to focus the holographic reconstruction on a playback plane into the near field.

Although different embodiments and groups of embodiments may be described separately in the detailed description that follows, any feature of any embodiment or group of embodiments may be combined with any other feature or combination of features of any other embodiment or group of embodiments. That is, all possible combinations and permutations of the features described in this disclosure are contemplated, provided they are contemplated by the appended claims.

Brief description of the drawings

Specific embodiments are described by way of example only with reference to the following figures:

Figure 1 is a schematic showing a reflective SLM producing a holographic reconstruction on a display;

Figure 2A illustrates a first iteration of an example Gerchberg-Saxton type algorithm;

Figure 2B illustrates the second and subsequent iterations of the example Gerchberg-Saxton type algorithm; Figure 2C illustrates alternative second and subsequent iterations of the example Gerchberg-Saxton type algorithm;

Figure 3 is a schematic of a reflective SLM LCOS;

Figure 4 shows the angular content of a virtual image effectively propagating from a display device to an aperture;

Figure 5A shows a vision system with a relatively small propagation distance;

Figure 5B shows a vision system with a relatively large propagation distance;

Figure 6A shows a vision system with a relatively large propagation distance, including a waveguide, for forming a virtual image at infinity;

Figure 6B shows an enlarged view of the optical paths of Figure 6A;

Figure 7A shows an image comprising a plurality of image areas (bottom) and a corresponding hologram comprising a plurality of hologram components (top);

Figure 7B shows a hologram, according to the present disclosure, characterized by routing or channeling holographically encoded light into a plurality of discrete hologram channels;

Figure 7C shows an optimized system arranged to route the light content of each channel of the hologram through a different optical path to the eye;

Figure 8 shows a system including a waveguide outputting multiple instances of an angular light channel;

Figure 9 shows a waveguide and vision system;

Figure 10 comprises a graph showing a relationship between a light ray angle and position along the waveguide (P<wg>) for the waveguide of Figure 9;

Figure 11 shows a control device according to embodiments, arranged relative to the graph of Figure 10, for an image having 4 zones;

Figure 12 shows an activation scheme for modifying the configuration of a control device comprising a light shutter device, according to embodiments;

Figure 13 shows a block diagram of a system for driving the light shutter device of Figure 12, according to embodiments.

The same reference numbers shall be used throughout the drawings to refer to the same or similar parts.

Detailed description of realizations

The present invention is not limited to the embodiments described below, but rather extends to the full scope of the appended claims. That is, the present invention may be implemented in various forms and should not be construed as limited to the described embodiments, which are presented for illustrative purposes only.

Singular terms may include plural forms unless otherwise specified.

A structure described as being formed on a top/bottom portion of another structure or on/below the other structure should be interpreted as including a case where the structures come into contact with each other and, in addition, a case where a third structure is arranged between them.

When describing a temporal relationship—for example, when the temporal order of events is described as "after," "subsequent," "next," "before," or the like—this disclosure should be considered to include both continuous and discontinuous events, unless otherwise specified. For example, the description should include an instance that is not continuous, unless terms such as "just," "immediate," or "direct" are used.

Although the terms "first," "second," etc., may be used herein to describe various elements, they are not limited to these terms. These terms are used only to distinguish one element from another. For example, a first element could be referred to as a second element, and similarly, a second element could be referred to as a first element, without departing from the scope of the appended claims.

The features of different embodiments may be partially or fully coupled or combined with each other, and may interoperate with each other in various ways. Some embodiments may be implemented independently or jointly in a codependent relationship.

Optical configuration

Figure 1 shows an embodiment in which a computer-generated hologram is encoded in a single spatial light modulator. The computer-generated hologram is a Fourier transform of the object for its reconstruction. It should be noted that this is only an example, and that other methods for generating the computer hologram are contemplated in the present disclosure. Therefore, the hologram can be said to be a representation of the object in the Fourier domain, the frequency domain, or the spectral domain. In this embodiment, the spatial light modulator is a reflective liquid crystal on silicon (LCOS) device. The hologram is encoded in the spatial light modulator, and a holographic reconstruction is formed on a reproduction field, for example, a light-receiving surface, such as a screen or diffuser.

A light source 110, for example a laser or laser diode, is arranged to illuminate the SLM 140 via a collimating lens 111. This collimating lens generates a generally flat wavefront of light incident upon the SLM. In Figure 1, the direction of the wavefront is abnormal (for example, two or three degrees below orthogonality to the plane of the transparent layer). However, in other embodiments, the generally flat wavefront is provided at normal incidence and a beam splitter arrangement is used to separate the input and output optical paths. In the embodiment shown in Figure 1, the arrangement is such that light from the light source reflects off a mirrored rear surface of the SLM and interacts with a light modulating layer to form an output wavefront 112. This output wavefront 112 is applied to an optical system including a Fourier transform lens 120, the focus of which is on a display 125. More specifically, the Fourier transform lens 120 receives a modulated light beam from the SLM 140 and performs a space-frequency transformation to produce a holographic reconstruction on the display 125.

It's worth noting that, in this type of holography, each pixel of the hologram contributes to the entire reconstruction. There is no one-to-one correlation between specific points (or image pixels) in the playback field and specific light-modulating elements (or hologram pixels). In other words, the modulated light exiting the light-modulating layer is distributed throughout the entire playback field.

In these embodiments, the position of the holographic reconstruction in space is determined by the dioptric (focusing) power of the Fourier transform lens. In the embodiment shown in Figure 1, the Fourier transform lens is a physical lens. That is, it is an optical Fourier transform lens, and the Fourier transform is performed optically. Any lens can act as a Fourier transform lens, but its performance will limit the accuracy of the Fourier transform it performs. One skilled in the art understands how to use a lens to perform an optical Fourier transform.

Hologram calculation

In some embodiments, the computer-generated hologram is a Fourier transform hologram, or simply a Fourier hologram or a Fourier-based hologram, in which a far-field image is reconstructed using the Fourier transform properties of a positive lens. The Fourier hologram is calculated by Fourier transforming the desired light field at the reproduction plane to the lens plane. Computer-generated Fourier holograms may be calculated using Fourier transforms.

A Fourier transform hologram may be calculated using an algorithm such as the Gerchberg-Saxton algorithm. Furthermore, this algorithm may be used to calculate a Fourier domain hologram (i.e., a Fourier transform hologram) from amplitude information in the spatial domain (such as a photograph). Phase information related to the object is efficiently recovered from the amplitude information in the spatial domain. In some embodiments, a computer-generated hologram is calculated from amplitude information using the Gerchberg-Saxton algorithm or a variant thereof.

The Gerchberg-Saxton algorithm considers the situation when the intensity cross sections of a light beam, Ia(x, y) and Ib(x, y), in planes A and B respectively, are known, and Ia(x, y) and Ib(x, y) are related by a single Fourier transform. With the intensity cross sections given, an approximation to the phase distribution in planes A and B is found, ^<a>(<x>, y) and Y<b>(<x>, y) respectively.

The Gerchberg-Saxton algorithm finds solutions to this problem following an iterative process. More specifically, the Gerchberg-Saxton algorithm iteratively applies spatial and spectral constraints while repeatedly transferring a data set (amplitude and phase), representative of Ia(x, y) and Ib(x, y), between the spatial domain and the Fourier (spectral or frequency) domain. The corresponding computer-generated hologram in the spectral domain is obtained through at least one iteration of the algorithm. The algorithm is convergent and is set to generate a hologram representing an input image. The hologram can be amplitude-only, phase-only, or fully complex.

In some embodiments, a phase-only hologram is calculated using an algorithm based on the Gerchberg-Saxton algorithm, as described in UK Patents 2,498,170 or 2,501,112, which are incorporated herein in their entireties by reference. However, the embodiments described herein describe the calculation of a phase-only hologram by way of example only. In these embodiments, the Gerchberg-Saxton algorithm recovers the phase information ^[u, v] from the Fourier transform of the data set, which generates known amplitude information T[x, y], wherein the amplitude information T[x, y] is representative of a target image (e.g., a photograph). Since magnitude and phase are intrinsically combined in the Fourier transform, the transformed magnitude and phase contain useful information about the accuracy of the calculated data set. Therefore, the algorithm can be used iteratively with feedback of both amplitude and phase information. However, in these embodiments, only the phase information ^[u, v] is used as a hologram to generate a holographic representation of the target image in an image plane. The hologram is a data set (e.g., a 2D cluster) of phase values.

In other embodiments, an algorithm based on the Gerchberg-Saxton algorithm is used to calculate a fully complex hologram. A fully complex hologram is a hologram that has a magnitude component and a phase component. The hologram is a data set (e.g., a 2D array) comprising a grouping of complex data values, wherein each complex data value comprises a magnitude component and a phase component.

In some embodiments, the algorithm processes complex data, and the Fourier transforms are complex Fourier transforms. The complex data may be considered to comprise (i) a real component and an imaginary component, or (ii) a magnitude component and a phase component. In some embodiments, the two components of the complex data are processed differently at various stages of the algorithm.

Figure 2A illustrates the first iteration of an algorithm, according to some embodiments, for calculating a phase-only hologram. The input to the algorithm is an input image 210 comprising a 2D array of pixels or data values, wherein each pixel or data value is a magnitude or amplitude value. That is, each pixel or data value in the input image 210 does not have a phase component. Therefore, the input image 210 may be considered a magnitude-only, amplitude-only, or intensity-only distribution. An example of such an input image 210 is a photograph or a video frame comprising a temporal sequence of frames. The first iteration of the algorithm begins with a data formation step 202A of assigning a random phase value to each pixel in the input image, using a random phase distribution (or random phase seed) 230, to form an initial complex data set, wherein each data element in the set comprises magnitude and phase. It can be said that the initial complex data set is representative of the input image in the spatial domain.

The first processing block 250 receives the initial complex data set and performs a complex Fourier transform to form a Fourier transformed complex data set. The second processing block 253 receives the Fourier transformed complex data set and generates a hologram 280A. In some embodiments, the hologram 280A is a phase-only hologram. In these embodiments, the second processing block 253 quantizes each phase value and sets each amplitude value to unity to form the hologram 280A. Each phase value is quantized according to the phase levels that can be represented in the pixels of the spatial light modulator that will be used to "display" the phase-only hologram. For example, if each pixel of the spatial light modulator provides 256 different phase levels, each phase value of the hologram is quantized into one phase level of the 256 possible phase levels. Hologram 280A is a phase-only Fourier hologram representing an input image. In other embodiments, hologram 280A is a fully complex hologram comprising a grouping of complex data values (each having an amplitude component and a phase component) derived from the received complex data set via the Fourier transform. In some embodiments, second processing block 253 constrains each complex data value to one of a plurality of allowable complex modulation levels to form hologram 280A. This constraining may include adjusting each complex data value to the nearest allowable complex modulation level in the complex plane.

Hologram 280A can be said to represent the input image in the spectral, Fourier, or frequency domain. In some embodiments, the algorithm stops at this point.

However, in other embodiments, the algorithm continues as represented by the dotted arrow in Figure 2A. In other words, the steps following the dotted arrow in Figure 2A are optional (i.e., not essential for all embodiments).

The third processing block 256 receives the modified complex data set from the second processing block 253 and performs an inverse Fourier transform to generate an inverse Fourier transformed complex data set. This inverse Fourier transformed complex data set is representative of the input image in the spatial domain.

The fourth processing block 259 receives the inverse Fourier transform complex data set and extracts the magnitude value distribution 211A and the phase value distribution 213A. Optionally, the fourth processing block 259 evaluates the magnitude value distribution 211A. Specifically, the fourth processing block 259 may compare the magnitude value distribution 211A of the inverse Fourier transform complex data set to the input image 510, which is, of course, a magnitude value distribution. If the difference between the magnitude value distribution 211A and the input image 210 is sufficiently small, the fourth processing block 259 may determine that the hologram 280A is acceptable. That is, if the difference between the distribution of magnitude values 211A and the input image 210 is sufficiently small, the fourth processing block 259 may determine that the hologram 280A is a sufficiently accurate representative of the input image 210. In some embodiments, the distribution of phase values 213A of the inverse Fourier transform complex data set is ignored for comparison purposes. It will be appreciated that any number of different methods may be employed to compare the distribution of magnitude values 211A and the input image 210 and that the present disclosure is not limited to any particular method. In some embodiments, a root mean square difference is calculated and if it is less than a threshold value, the hologram 280A is deemed acceptable. If the fourth processing block 259 determines that the hologram 280A is not acceptable, a further iteration of the algorithm may be performed. However, this comparison step is not essential and in other embodiments, the number of iterations of the algorithm performed is predetermined or preset or defined by the user.

Figure 2B depicts a second iteration of the algorithm and any subsequent iterations. The phase value distribution 213A from the previous iteration is fed back through the processing blocks of the algorithm. The magnitude value distribution 211A is rejected in favor of the magnitude value distribution of the input image 210. In the first iteration, the data training stage 202A generated the first complex data set by combining the magnitude value distribution of the input image 210 with a random phase distribution 230. However, in the second and subsequent iterations, the data training stage 202B comprises forming a complex data set by combining (i) the phase value distribution 213A from the previous iteration of the algorithm with (ii) the magnitude value distribution of the input image 210.

The complex data set formed in the data formation step 202B of Figure 2B is processed in the same manner as in Figure 2A to form the second iteration hologram 280B. Therefore, the explanation of the process will not be repeated here. The algorithm may stop after the second iteration hologram 280B has been calculated. However, any number of additional iterations of the algorithm may be performed. It should be understood that the third processing block 256 is only necessary if the fourth processing block 259 or an additional iteration is required. The output hologram 280B generally improves with each iteration. However, in practice, a point is often reached where no measurable improvement is observed or where the benefit of performing an additional iteration is offset by the negative effect of the additional processing time. Therefore, the algorithm is described as iterative and convergent.

Figure 2C depicts an alternative embodiment of the second and subsequent iterations. The phase value distribution 213A from the previous iteration is fed back through the algorithm's processing blocks. The magnitude value distribution 211A is rejected in favor of an alternative magnitude value distribution. In this alternative embodiment, the alternative magnitude value distribution is derived from the magnitude value distribution 211 from the previous iteration. Specifically, the processing block 258 subtracts the magnitude value distribution of the input image 210 from the magnitude value distribution 211 from the previous iteration, scales the difference by a gain factor to the input image 210, and subtracts the scaled difference from the input image 210. This is expressed mathematically by the following equations, where the subscripted text and numbers indicate the iteration number:

Rn<+1>[x, y] = F' {exp(¡Yn[u, v]}

Yn [u, v] = zF{r| • exp(izRn [x, y]}

n = T[x, y] - a(|Rn [x, y]|- T[x, y])

where:

F' is the inverse Fourier transform;

F is the forward Fourier transform;

R[x, y] is the complex data set generated by the third processing block 256;

T[x, y] is the input or destination image;

z is the phase component;

^ is the 280B phase-only hologram;

n is the new distribution of magnitude values 211B; and

a is the gain factor.

The gain factor α can be fixed or variable. In some embodiments, the gain factor α is determined based on the size and speed of the incoming destination image data. In some embodiments, the gain factor α depends on the number of iterations. In some embodiments, the gain factor α is a sole function of the number of iterations.

The embodiment of Figure 2C is identical to that of Figures 2A and 2B in all other respects. It should be noted that the phase-only hologram ^(u , v) comprises a phase distribution in the frequency or Fourier domain.

In some embodiments, the Fourier transform is performed by the spatial light modulator. Specifically, the hologram data is combined with second data that provides optical power. That is, the data written to the spatial light modulator comprises hologram data representing the object and lens data representative of a lens. When displayed on a spatial light modulator and illuminated with light, the lens data emulates a physical lens; that is, it focuses light in the same manner as corresponding physical optics. Therefore, the lens data provides optical or focusing power. In these embodiments, the physical lens may be omitted from the Fourier transform 120 of Figure 1. It is known how to calculate data representative of a lens. Data representative of a lens may be referred to as a "software lens." For example, a phase-only lens may be formed by calculating the phase delay caused by each point on the lens due to its refractive index and its spatially varying optical path length. For example, the optical path length at the center of a convex lens is greater than the optical path length at the edges of the lens. An amplitude-only lens may be formed by a Fresnel zone plate. In computer-generated holography, it is also known to combine data representative of a lens with a hologram to perform a Fourier transform of the hologram without the need for a physical Fourier lens. In some embodiments, the lens data is combined with the hologram by simple addition, such as simple vector addition. In some embodiments, a physical lens is used in conjunction with a software lens to perform the Fourier transform. Alternatively, in other embodiments, the Fourier transform lens is omitted entirely, so that the holographic reconstruction is performed in the far field. In other embodiments, the hologram may be combined in the same manner with grating data, i.e., data arranged to perform the function of a grating, such as image control. Again, it is known in the field how to calculate such data. For example, a phase-only grating can be formed by modeling the phase delay caused by each point on the surface of a flame grating. An amplitude-only grating can simply be superimposed on an amplitude-only hologram to provide angular direction to the holographic reconstruction. The second data providing the lensing and/or direction may be referred to as a light-processing function or light-processing pattern to distinguish it from the hologram data, which may be referred to as an imaging function or imaging pattern.

In some embodiments, the Fourier transform is performed jointly by a physical Fourier transform lens and a software lens. That is, a portion of the optical power contributing to the Fourier transform is provided by a software lens, and the remainder of the optical power contributing to the Fourier transform is provided by one or more physical optics.

In some embodiments, a real-time engine is provided, arranged to receive image data and calculate holograms in real time using the algorithm. In some embodiments, the image data is a video comprising a sequence of frames. In other embodiments, the holograms are pre-calculated, stored in computer memory, and retrieved as needed for display in a SLM. That is, in some embodiments, a repository of predetermined holograms is provided.

Embodiments relate to Fourier holography and Gerchberg-Saxton type algorithms by way of example only. The present disclosure is equally applicable to Fresnel holography and Fresnel holograms, which may be calculated by a similar method. The present disclosure is also applicable to holograms calculated by other techniques, such as those based on point cloud methods. As will be appreciated, the Figures below herein are described as comprising a point cloud method for calculating holograms. However, other methods of calculating holograms may be used, including the Fourier method described above in connection with Figures 2A through 2C.

Light modulation

A spatial light modulator can be used to display the diffractive pattern, including the computer-generated hologram. If the hologram is phase-only, a spatial light modulator that modulates the phase is required. If the hologram is a fully complex hologram, a spatial light modulator that modulates both phase and amplitude can be used, or a first spatial light modulator that modulates the phase and a second spatial light modulator that modulates the amplitude can be used.

In some embodiments, the light modulating elements (i.e., pixels) of the spatial light modulator are cells containing liquid crystal. That is, in some embodiments, the spatial light modulator is a liquid crystal device whose optically active component is the liquid crystal. Each liquid crystal cell is configured to selectively provide a plurality of light modulation levels. That is, each liquid crystal cell is configured to operate at any given time at a light modulation level selected from a plurality of possible levels. Each liquid crystal cell can be dynamically reconfigured to a different light modulation level from the plurality of available levels. In some embodiments, the spatial light modulator is a reflective liquid crystal on silicon (LCOS) spatial light modulator, but the present disclosure is not limited to this type of modulator.

An LCOS device provides a dense arrangement of light-modulating elements, or pixels, within a small aperture (e.g., a few centimeters across). The pixels are typically about 10 micrometers or less in size, resulting in a diffraction angle of a few degrees, which means the optical system can be compact. It is easier to adequately illuminate the small aperture of an SLM LCOS than the larger aperture of other liquid crystal devices. An LCOS device is usually reflective, meaning the circuitry controlling the pixels in an SLM LCOS can be buried beneath the reflective surface. This results in a larger aperture ratio. In other words, the pixels are densely packed, meaning there is very little dead space between them. This is advantageous because it reduces optical noise in the playback field. An SLM LCOS uses a silicon base plate, which has the advantage of making the pixels optically flat. This is particularly important for a phase-modulation device.

A suitable exemplary SLM LCOS is described below with reference to Figure 3. An LCOS device is formed using a monocrystalline silicon substrate 302. It comprises a two-dimensional array of flat, square aluminum electrodes 301, separated by a gap 301a, disposed on the top surface of the substrate. Each of the electrodes 301 can be addressed by circuit 302a integrated in the substrate 302. Each of the electrodes forms its respective planar mirror. An alignment layer 303 is disposed over the electrode array, and a liquid crystal layer 304 is disposed over the alignment layer 303. A second alignment layer 305 is disposed over a planar transparent layer 306, e.g., glass. A single transparent electrode 307, e.g., ITO, is disposed between the transparent layer 306 and the second alignment layer 305.

Each of the square electrodes 301 defines, together with the overlapping region of the transparent electrode 307 and the intervening liquid crystal material, a controllable phase modulating element 308, often referred to as a pixel. The effective area of the pixel, or fill factor, is the percentage of the total pixel that is optically active, considering the spacing between the pixels 301a. By controlling the voltage applied to each electrode 301 with respect to the transparent electrode 307, the properties of the liquid crystal material of the respective phase modulating element can be varied, thereby providing a variable delay to the light incident thereon. The effect is to provide phase modulation only to the wavefront, i.e., with no amplitude effect.

The described LCOS spatial light modulator outputs light that is spatially modulated by reflection. Reflective LCOS spatial light modulators have the advantage that the signal lines, gate lines, and transistors are located below the reflected surface, resulting in high fill factors (typically greater than 90%) and high resolutions. Another advantage of using a reflective LCOS spatial light modulator is that the liquid crystal layer can be half the thickness that would be required with a transmissive device. This considerably improves the switching speed of the liquid crystal (a key advantage for the projection of moving video images). However, the teachings of the present disclosure can also be implemented using a transmissive LCOS spatial light modulator.

Projecting images using a small display device and a long viewing distance

The present disclosure relates to image projection wherein the separation between the display device and the observer is much greater than the size of the display device. The viewing distance (i.e., the distance between the observer and the display device) may be at least one order of magnitude greater than the size of the display device. The viewing distance may be at least two orders of magnitude greater than the size of the display device. For example, the pixel area of the display device may be 10 mm x 10 mm and the viewing distance may be 1 m. The image projected by the system is formed on a display plane spatially separated from the display device.

According to the present disclosure, the image is formed by holographic projection. A hologram is displayed on the display device. The hologram is illuminated by a light source (not shown), and an image is perceived on a display plane spatially separated from the hologram. The image may be real or virtual. For the following discussion, it is useful to consider a virtual image formed upstream of the display device. That is, one that appears behind the display device. However, it is not essential that the image be virtual, and the present disclosure is equally applicable to a real image formed between the display device and the vision system.

The display device comprises pixels that show the hologram. The pixel structure of the display device is diffractive. Therefore, the size of the holographic image is governed by the laws of diffraction. A consequence of the diffractive nature of the display device is explained below, with reference to Figure 4.

Figure 4 shows a pixelated display device 402 arranged to display a hologram forming a virtual image 401 upstream of the display device 402. The diffraction angle, q, of the display device determines the size of the virtual image 401. The virtual image 401, the display device 402 and the vision system 405 are arranged about an optical axis, Ax.

The vision system 405 has an entrance opening 404 and a plane of view 406. The vision system 405 may be a human eye. Thus, the entrance opening 404 may be the pupil and the plane of view 406 may be the retina.

The light traveling between the display device 402 and the vision system 405 is modulated with a hologram of the image (not the image itself). However, Figure 4 illustrates how the hologram divides the virtual image content by angle. Each illustrated ray beam relates to a different portion of the virtual image 401. More specifically, the light in each ray beam is encoded by the hologram with information about a portion of the virtual image. Figure 4 shows five examples of ray beams, each characterized by a respective angle with the optical axis, Ax, and each representing a portion of the virtual image. In this example, one of the light beams passes through the pupil 404, and the other four light beams are blocked by the pupil 404. Again, the five different ray beams correspond to five different portions of the virtual image 401. The entire virtual image content is effectively divided by angle. The light beam traveling along the optical axis, Ax, carries the central portion of the image information, i.e., information relative to the center of the image. The other light beams carry the other portions of the information. The two light beams shown at the ends of the light cone carry the edge portions of the image information. One consequence of this division of image information by angle is that not all of the image content can pass through the input aperture 404 of the vision system at a given viewing position. In other words, the eye does not receive all of the image content. In the example of Figure 4, only one of the five illustrated light beams passes through the pupil 404 at any viewing position. The reader will understand that the five light beams are shown for exemplary purposes only and that the process described is not limited to the division of the virtual image information into just five light beams.

In this example, the eye receives the central portion of the image information. The pupil of the eye blocks the edge portion of the image information. The reader will understand that if the observer moves up or down, the eye may receive a different beam of light and, for example, block the central portion of the image information. Therefore, the observer only sees part of the entire image. The remaining image information is blocked by the entrance pupil. The observer's vision is greatly restricted, as they are actually viewing the image through the small opening of the display device itself.

In summary, light propagates in the range of diffraction angles from the display device. At a viewing distance of 1 m, only a small range of angles from the display device can propagate through the pupil to form an image on the retina for a given eye position. The only visible parts of the virtual image are those within the small angular range shown in Figure 4 and passing through the entrance aperture. Consequently, the field of view is very small, and the specific angular range depends strongly on the eye position.

The problem of the small field of view and sensitivity to eye position, explained in Figure 4, is due to the large viewing distance and the small aperture of the display device. The importance of viewing distance is explained in more detail in Figures 5 through 7.

Figure 5A shows a display device 502 arranged to display a hologram and propagate modulated light accordingly to a viewing system comprising an input aperture 504 and a viewing plane 506. The virtual image 501 is at infinity, so rays drawn between the virtual image and the display device are collimated. The lower portion of Figure 5A shows an enlarged view of the viewing system. This figure is schematic, so physiological details of the eye are not shown. In practice, there is, of course, a light source (not shown in Figure 5A) arranged to illuminate the display device 502.

Figure 5A only shows the light rays that can propagate through the aperture 504; other rays that cannot pass through the aperture 504 are omitted. However, it is understood that, in practice, these other rays would also propagate from the display device 502. In Figure 5A, the distance between the display device and the viewing plane is small enough such that the full diffraction angle of the display device can form the image on the retina. All light propagation paths shown from the virtual image pass through the input aperture. Accordingly, all virtual image points are projected onto the retina and the entire image content is delivered into the viewing plane. The field of view of the perceived image is therefore maximum. In the optimal position, the field of view is equal to the diffraction angle of the display device. Interestingly, different image points on the retina are formed from light propagating from different regions of the display device 502; For example, the image point closest to the top in Figure 5A is formed from light propagating only from the bottom of the display device. Light propagating from other regions of the display device does not contribute to this image point.

Figure 5B shows the situation that arises as the viewing distance increases.

In more detail, Figure 5B shows a display device 502' arranged to display a hologram and propagate modulated light accordingly to a viewing system comprising an input aperture 504' and a viewing plane 506'. The virtual image 501' is at infinity, so rays drawn between the virtual image 501' and the display device 502' are collimated. The lower portion of Figure 5B shows an enlarged view of the viewing system. This figure is schematic, so physiological details of the eye are not shown. In practice, there is, of course, a light source (not shown in Figure 5B) arranged to illuminate the display device 502'.

Figure 5B shows only the light rays that can propagate through the opening 504'. At the larger viewing distance of Figure 5B, some ray bundles are blocked by the entrance opening 504'. Specifically, ray bundles associated with the edge portions of the virtual image are blocked by the entrance pupil 504'. Consequently, the entire virtual image is not visible, and the visible portion is highly dependent on the eye position. Therefore, large distances between the display device and the vision system are problematic due to the small size of the display device.

Figure 6A shows an improved system comprising a display device 602, which propagates light encoded with a hologram displayed on the device 602 towards a vision system comprising an entrance aperture 604 and a vision plane 606. In practice, there is, of course, a light source (not shown) arranged to illuminate the display device 602. The improved system further comprises a waveguide 608 positioned between the display device 602 and the entrance aperture 604. The lower part of Figure 6A shows an enlarged view of the entrance pupil 604 and the vision plane 606. This figure is schematic, so the physiological details of the eye are not shown.

The viewing distance in Figure 6A is the same as that in Figure 5B. However, the waveguide 608 efficiently recovers the blocked beams in Figure 5B, so that the vision system receives all of the image information, despite the longer viewing distance.

The presence of the waveguide 608 allows the viewing eye to capture the entire angular content of the display device 602, even at this relatively large projection distance. This is because the waveguide 608 acts as a pupil expander, in a well-known manner, as briefly described herein.

In summary, the waveguide 608 comprises a substantially elongated structure. In this example, it comprises an optical plate of refractive material, but other types of waveguides are also known and may be used. The waveguide 608 is positioned such that it intersects the cone of light projected from the display device 602, for example, at a forced angle. The size, location, and position of the waveguide 608 are configured to ensure that light from each of the five ray bundles within the light cone enters the waveguide 608. Light from the light cone enters the waveguide 608 through its first planar surface 610 (located closest to the display device 602) and is guided, at least partially, along the waveguide 608, before being emitted through its second planar surface 612, substantially opposite the first surface 610 (located closest to the eye). As will be well understood, the second flat surface 612 is partially reflective and partially transmissive. In other words, as each light ray travels, within the waveguide 608, from the first planar surface 610 to the second planar surface 612 of the waveguide 608, some of the light is transmitted out of the waveguide 608 and some is reflected off the second planar surface 612, back to the first planar surface 610. The first planar surface 610 is reflective, such that all light striking it from within the waveguide 608 is reflected back toward the second planar surface 612. Thus, some of the light may simply be refracted between the two planar surfaces 610, 612 of the waveguide 608 before being transmitted, while some of the light may be reflected and therefore experience one or more reflections (or bounces) between the planar surfaces 610, 612 of the waveguide 608, before being transmitted. A net effect of the waveguide 608 is that light transmission is effectively spread across multiple locations on the second planar surface 612 of the waveguide 608.

In this way, the entire angular output content of the display device 602 can be present at a greater number of positions in the viewing plane (and at a greater number of positions in the aperture plane) than in the absence of the waveguide 608. This means that light from each ray bundle can enter the input aperture 604 and contribute to the image formed by the viewing plane 606, despite the relatively large projection distance. In other words, the entire angular content of the display device 602 can be captured by the eye. Therefore, the entire diffraction angle of the display device 602 is utilized, and the viewing window for the user is maximized. In turn, this means that all light rays contribute to the perceived virtual image 601.

Figure 6B shows the individual optical paths of each of the five ray bundles contributing to five respective image points within the virtual image 601 formed in Figure 6A, labeled from top to bottom as R1 to R5, respectively. As can be seen therein, light from R1 and R2 is refracted and then transmitted by the waveguide 608. Light from R4, on the other hand, undergoes a single bounce before being transmitted. Light from R3 comprises part of the light from a corresponding first part of the display device 602, which is simply refracted by the waveguide 608 before being transmitted, and part of the light from a corresponding second, different part of the display device 602, which undergoes a single bounce before being transmitted. Similarly, the light from R5 comprises part of the light from a corresponding first portion of the display device 602, which experiences a single bounce before being transmitted, and part of the light from a corresponding second, different portion of the display device 602, which experiences two bounces before being transmitted. For each of R3 and R5, two different portions of the LCOS propagate light corresponding to that portion of the virtual image.

The inventors have recognized that light from different portions of the virtual image 601 (i.e., different points in the virtual image) follows different optical paths through the system. In one embodiment illustrated in Figures 7A and 7B, the inventors have configured the system such that, in simple terms, (i) the virtual image comprises a plurality of discrete virtual image components or areas, and (ii) light from each virtual image component is associated with a different number of bounces/reflections within the waveguide 708.

Figure 7A shows an image 752 for projection comprising eight image areas/components, V1 through V8. Figure 7A shows eight image components by way of example only, and the image 752 may be divided into any number of components. Figure 7A also shows the encoded light pattern 754 that may reconstruct the image 752, for example, by being transformed by the lens of a suitable vision system. The encoded light pattern 754 comprises the first through eighth subholograms or components, H1 through H8, corresponding to the first through eighth image components/areas, V1 through V8. Figure 7A further shows how a hologram calculated in accordance with this disclosure effectively decomposes image content by angle. Thus, the hologram may be characterized by its light channeling. This is illustrated in Figure 7B. Specifically, the hologram in accordance with this disclosure directs light to a plurality of discrete areas. In the example shown, the discrete areas are discs, but other shapes are conceivable. After propagation through the waveguide, the size and shape of the optimal disc can be related to the size and shape of the entrance pupil of the vision system. This light channeling only occurs thanks to the specific hologram determination method disclosed herein.

Figure 7C shows an improved vision system 700, in accordance with the embodiments illustrated in Figures 7A and 15B. GB2108456.1 describes methods for deriving a data structure (as shown in Figures 13 and 14), which may be applied to the schematic illustrated in Figures 7A and 7B.

The vision system 700 comprises a display device, which in this arrangement includes an LCOS 702. The LCOS 702 is arranged to display a modulation pattern (or "diffractive pattern") comprising the hologram and to project holographically encoded light towards an eye 705 comprising a pupil acting as an aperture 704, a lens 709 and a retina (not shown) acting as the plane of view. A light source (not shown) is arranged to illuminate the LCOS 702. The lens 709 of the eye 705 performs a hologram-to-image transformation.

The vision system 700 further comprises a waveguide 708 positioned between the LCOS 702 and the eye 705. The projection distance in Figure 7C may be relatively large. However, as described in connection with the previous Figures, the presence of the waveguide 708 allows the eye 705 to receive the full angular content of the LCOS 702, even at this relatively large projection distance. This is because the waveguide 708 acts as a pupil expander, in the manner described above.

Furthermore, in this arrangement, once the LCOS 702 has been encoded according to the methods described, the waveguide 708 may be angled relative to the LCOS 702 to establish a unique relationship between the light from the LCOS 702 and the virtual image that will be perceived by the observer. The size, location, and position of the waveguide 708 are configured to ensure that light from each portion of the virtual image enters the waveguide 708 and is guided along its elongated axis, bouncing between its substantially planar surfaces of the waveguide 708. Each time the light reaches the second planar surface (the one closest to the eye 705), some of the light is transmitted and some is reflected.

Figure 7C shows a total of nine bounce points, B0 through B8, along the waveguide 702. The reader will note that the center of the image 752 is kept white. Figure 7C shows the light reflection or bounce points, B0 through B8, zero through nine, within the waveguide. It is to be understood that the reflection points, B0 through B8, are also transmission points. Although light relative to all image points (V1-V8) is transmitted out of the waveguide at each bounce from the second planar surface of the waveguide 708, only light from one angular portion of the image (e.g., light from V1 through V8) has a path that allows it to reach the eye 705 from each bounce point, B0 through B8. Furthermore, light from a different angular portion of the image, V1 through V8, reaches the eye 705 from each bounce point. Figure 7C shows the light of all the different angular contents emitted at each bounce point (represented by a plurality of short arrows at each transmission point), but only shows the optical path, toward the eye 705, of the respective angular content that will actually reach the eye 705—and thus contribute to the corresponding part of the virtual image that the observer will perceive—from that part of the waveguide. For example, for bounce zero, B0, the light transmitted by the waveguide 708 is simply refracted and does not undergo any reflection. The light from the eighth subhologram, H8, reaches the eye from bounce zero, B0. For the next bounce, B1, the light transmitted by the waveguide 702 undergoes one bounce before transmission. The light from the seventh hologram, H7, reaches the eye from the next bounce, B1. This continues in sequence until the light transmitted by the waveguide 708 at the final bounce, B8, has bounced eight times before being transmitted and reaching the eye 705, and comprises light encoded according to the first hologram, H1.

In the example shown in Figures 7A-C, light from a single image area reaches the eye from each bounce point. Therefore, a spatial correlation is established between the virtual image areas and their associated bounce point on the waveguide when determining the hologram as described herein. In other examples, there may be relatively small overlaps, such that a region of the image originates from two adjacent transmission points and is therefore encompassed within two adjacent light discs propagating from the waveguide toward the viewing plane.

Thus, the recognitions made by the inventors, and the methods and arrangements described herein and in the co-pending GB2108456.1 patent, may enable a diffractive pattern (or "light modulation pattern") to be generated comprising a hologram which, when displayed on an LCOS or other suitable display device, may enable light to be emitted therefrom effectively in a plurality of "discs", or bundles of light rays, each of which corresponds to (more specifically, encodes) a different respective portion of the corresponding virtual image.

Therefore, improved methods and arrangements are described herein that enable holograms to be calculated and displayed on a suitable display device, such that the observer can see the images when the display device is illuminated with a suitable light source. By using the methods described in GB2108456.1, the images seen by the observer can be free of ghost images and brightened by the contribution of light, which would conventionally have contributed to a ghost image, rather than contributing to the main image.

The improved methods and arrangements described herein can be implemented in various display applications and systems. For example, they can be implemented in a head-up display (HUD). In an improvement over many conventional HUDs, in which virtual images are formed, the improved methods and arrangements described herein allow virtual images to be created at finite distances, which can be selected and adjusted by a suitable controller, while eliminating ghost images.

Although virtual images, which require the eye to transform the received modulated light to form a perceived image, have been discussed herein, the improved methods and arrangements described herein can be applied to real images.

Waveguide aperture

Although the previous Figures have shown a single eye or a single "hole" or "entrance pupil", all of the arrangements and methods described herein are applicable to a vision system with multiple entrance pupils, for example, and more commonly, to a binocular vision system such as a human observer having two eyes.

The inventors have recognized that, when calculating a hologram of a target image as described above, which includes constraining the hologram according to an entrance pupil of the viewing system, possible effects must be considered when the viewing system has multiple entrance pupils, for example, when the observer views the holographically reconstructed image with both eyes. In some embodiments, the hologram may be constrained according to one of the multiple entrance pupils, but part or all of the image may also be visible to one or more additional of the multiple entrance pupils. In some embodiments, the hologram may be constrained according to two or more of the multiple entrance pupils. For example, two sub-holograms, each of which is constrained according to a different respective entrance pupil, may be calculated and combined (e.g., summed) into a single hologram for display on a display device. For example, a left-eye hologram and a right-eye hologram may be calculated from a respective left-eye image and a right-eye image, wherein the left-eye hologram is constrained during the calculation according to the entrance pupil of the observer's left eye, and the right-eye hologram is constrained during the calculation according to the entrance pupil of the observer's right eye. In some embodiments, two or more holograms, each constrained according to a different one of the multiple entrance pupils, may be interlaced together. In other words, the two holograms may be displayed alternately, in rapid succession, such that the observer perceives the two corresponding images as having formed substantially simultaneously.

The inventors have recognized that additional guidance or control can be applied to the light propagating from the displayed hologram to a viewing system to make it more suitable for reception by multiple entrance pupils to that viewing system.

As described in detail above, when a hologram is calculated according to the methods described and viewed and propagated by a system such as that shown in Figure 7C, at each transmission point (or "bounce point", such as B0 to B8 as shown in Figure 7C), light corresponding to all image points (V1-V8) is transmitted out of the waveguide. The central image region 752 in Figure 7A is empty, so the holographic channel associated with the central image region is not shown or labeled in Figure 7B. Accordingly, in Figure 7C, the optical path associated with transmission point B4 may not deliver any image content to the viewing system. However, in a single entrance pupil vision system, only light from one angular portion of the image (e.g., light from each of the points V1 through V8) has a path that allows it to reach the eye 705, from each bounce point, B0 through B8. Furthermore, light from a different angular portion of the image, V1 through V8, reaches the eye 705 from each bounce point. Thus, in Figure 7C, a single entrance pupil 704 is shown receiving the entire angular content of an image, but each angular region of the image originates from a different bounce point in the waveguide 708. However, the present inventors have recognized that if a vision system has two or more entrance pupils, there is a risk that light from the same angular region of the image will reach both pupils simultaneously, since all angular regions of the image are emitted at each bounce point in the waveguide. For example, where the vision system is a human and multiple entrance pupils comprise both eyes, the right eye may receive light comprising particular angular content Vx via a light path that is emitted from the waveguide after a first number of bounces By, and the left eye may receive the same angular content Vx simultaneously (or, substantially simultaneously, given the speed of light) via a light path that is emitted from the waveguide after a second, different number of bounces B. This can lead to confusion since the human brain, and, indeed, a processor associated with any non-human vision system, does not expect to receive the same portion of an image (i.e., the same image content), at the same angle, simultaneously in two entrance pupils that are spatially separated from one another. Instead, the brain (or other processor) would expect light from a single point, or a single portion of an image, to be received by the two (or more) entrance pupils at different respective angles.

Figure 8 illustrates the problem described above, addressed by the present inventors. Figure 8 shows a display device 802 displaying a hologram calculated according to the methods described in GB2108456.1.

The display device 802 is illuminated by a light source (not shown). The light is spatially modulated by the displayed hologram and propagates toward a waveguide 804. As described in detail in connection with the preceding figures, the light is refracted within the waveguide 804. Upon refraction, some of the light is emitted toward the observer at a first transmission point (or "bounce point"), and other portions of the light are reflected (or "bounced") internally within the waveguide 804 prior to transmission, each being emitted toward the observer at different transmission points (or "bounce points") along the waveguide 804.

Figure 8 only shows the optical paths 810 for light from one portion (Vx) of the image represented by the displayed hologram. In practice, light from other portions of the image would also be transmitted from the waveguide toward the observer, but has been omitted from Figure 8 for ease of understanding. It should be noted that light from other angular portions of the image would follow different optical paths between the bounce points of the waveguide 804 and the observer, and that light from each angular portion of the image would be emitted at each bounce point of the waveguide toward the observer.

Figure 8 shows an entrance pupil plane 812, indicating the plane where the observer's pupils are located, and an image display plane 814, indicating the plane where the image is formed on the observer's retinas. Figure 8 also shows a first marker 808 indicating which of the optical paths 810 is directed toward the observer's right eye and will enter through the entrance pupil, and a second marker 806 indicating which of the optical paths 810 is directed toward the observer's left eye and will enter through the entrance pupil. As can be seen, each eye will only receive light from one of the multiple optical paths 810, and each eye will receive light through a different optical path 810 than the other respective one, because the observer's eyes are naturally spatially separated from one another. However, as described above, all of the optical paths shown in Figure 8 comprise light from the same portion (i.e., a common portion) of the image. Therefore, both eyes will receive the same image content at the same angle of incidence, albeit through different optical paths. The present inventors have recognized that this can be confusing and affect the clarity of the image perceived by the observer. The inventors have addressed this potential problem, as is clear from the following description. Furthermore, although only the optical paths of light from one angular portion of the image are shown in Figure 8, it is possible that light from one or more angular portions of the image may also be received simultaneously by both the left and right eyes of the observer in a similar manner.

Figure 9 shows a display system, or light engine, including a waveguide 904 and a vision system 910 comprising two entrance pupils. In this example, the vision system 910 is a human observer and the two entrance pupils form part of the observer's left eye 906 and right eye 908, respectively, however, this should not be considered limiting and the present disclosure also applies to other types of display systems. Other portions of the display system are omitted from Figure 9. Waveguide 904 has a center point 902 defined on its longer face nearest viewing system 910, where the "position along waveguide" (P<wg>) is zero (Pwg = 0) at the center point 902. The optical axis is represented by a horizontal dashed line extending (in the "z" direction in this example) from the center point 902 to the viewing system 910. The "x" axis is defined by a vertical line extending from the center point 902 and substantially perpendicular to the optical axis. Waveguide 904 is arranged at an angle "α" to the x-axis. The entrance pupil plane, in which the observer's pupils are located, spatially separated from each other in the entrance pupil plane, is substantially parallel to the x-axis in this example. However, the present disclosure is not limited to the arrangement of Figure 9 or the relative positions of any of its elements. For example, in practice, an observer's eyes may not be precisely located in a common entrance pupillary plane perpendicular to an optical axis extending toward the observer from the waveguide. The location of the observer's eyes, at any given time, may be calculated or otherwise obtained by a processor integrated into or in communication with the display system. For example, any suitable eye-tracking method may be employed.

The "position along the waveguide" (Pwg) of other points on that waveguide face 904 may be defined by their position relative to the center point 902. By way of example, any point to the right of the center point 902, as viewed by observer 910 in Figure 9, may be assigned a positive (+) value of Pwg, the magnitude of which is defined by the distance along the waveguide face between the center point 902 and the other point. Conversely, any point to the left of the center point 902, as viewed by observer 910 in Figure 9, may be assigned a negative (-) value of P<wg>, the magnitude of which is defined, in turn, by the distance along the waveguide face between the center point 902 and the other point. A distance "D", substantially parallel to the optical axis, is defined between the center point 902 and the plane of the entrance pupil. This distance "D" may be relatively large compared to the size of the entrance pupils of the observer's eye and/or to the size of the display device (not shown) that displays the hologram of an image to be viewed by the vision system 910. For example, the distance "D" may be approximately 1000 millimeters (1000 mm).

It can be seen that the observer's eyes 906, 908 are, naturally, spatially separated from one another. The separation between the entrance pupils of the observer's eyes can be referred to as the "interpupillary distance" (IPD). In the example shown in Figure 9, the right eye's entrance pupil 908 is shifted relative to the intersection of the optical axis and the entrance pupil plane in a positive (+) direction along the x-axis, while the left eye's entrance pupil 906 is shifted relative to the intersection of the optical axis and the entrance pupil plane in a negative (-) direction along the x-axis. Again, this is only an example and is not limiting. The present inventors have recognized that control can be provided to account for the different respective positions of the observer's two eyes (and, consequently, the different respective locations of two or more entrance pupils within any multi-input pupil vision system), to ensure that no part of an image is received by both eyes at substantially the same time. As a reminder, the special type of hologram according to this disclosure effectively divides or separates the image content (albeit in the hologram domain) by angle.

Figure 10 comprises a graph illustrating the surveys made by the present inventors, applied to a display system or light engine, such as that shown in Figures 7C, 8 or 9. The graph shows the relationship between the position along the waveguide (P<wg>) of a point on the long face of the waveguide, oriented towards an observer or viewing system, and the angle from each eye to said point on the waveguide. In some embodiments, the waveguide is tilted with respect to the display device (i.e., the hologram) and/or the entrance pupil plane of the viewing system.

The graph in Figure 10 shows two lines: one, 1006, for the first entrance pupil of one viewing system, such as the observer's left eye, and the other, 1008, for a different second entrance pupil of the viewing system, such as the observer's right eye. As can be seen, for any given light angle, 0, there exists a first position along the waveguide, P<wg>1, from which light is emitted to reach the left eye. For that same light angle, 0, there exists a second, different position along the waveguide, Pwg2, from which light is emitted to reach the right eye. Thus, both eyes will receive the same image content (i.e., light angle) at virtually the same time, but from different parts of the waveguide. This is the cause of a problem addressed by the inventors.

All rays (or ray bundles) corresponding to different parts of the image are emitted from multiple locations (i.e., from multiple "bounce points") in the waveguide. Therefore, if two or more spatially modulated light rays, emitted from the waveguide from different positions but at the same angle, enter both entrance pupils virtually simultaneously, the image content received by both entrance pupils (i.e., by both eyes of the observer) will be the same. This can confuse the observer and affect the quality of the image they see or perceive.

Therefore, the present inventors have recognized that, according to embodiments, the light received by each entrance pupil of a multi-pupil input vision system must be controlled. For example, the simultaneous reception, by two or more entrance pupils of a multi-pupil input vision system, of light with the same image content and the same angle must be reduced and, at least in some cases, eliminated. Furthermore, the inventors have recognized that, at least in some cases, two or more entrance pupils of a multi-pupil input vision system must be prevented from simultaneously receiving light with the same image content (i.e., light in the same angle range). The inventors' solution will be understood in relation to Figure 11, which also shows first lines 1106 and second lines 1108 representing light received by the left and right eyes, respectively, of an observer according to one example.

Each angle in Figure 11 corresponds to a different part of the image due to the special type of hologram employed in accordance with this disclosure. In summary, the inventors have identified, from their analysis of this optical geometry, that a plurality of angular ranges or "zones" (i.e., sub-areas) of the waveguide can be identified, in which only one of the eyes will receive the corresponding image content. For example, in zone 2 of the waveguide, the angular range of light received by the left eye does not overlap with the angular range of light received by the right eye. In any zone, the maximum angle received by one eye is less than the minimum angle received by the other, so that there is no overlap. However, the zones are perfectly connected, so that all image content (i.e., all angles) is delivered to the vision system. Looking at this from another perspective, the angle range associated with zone 2 in Figure 11 is delivered to the first eye from a first subarea (i.e., first P<wg> range) of the waveguide that does not overlap (but seamlessly connects) to a second subarea (i.e., second Pwg range) of the waveguide that delivers the same angular content to the second eye. The end of the first subarea (e.g., the upper Pwg boundary for the first eye) is immediately adjacent (i.e., seamlessly connects) to the start of the second subarea (e.g., the lower Pwg boundary for the other eye), or vice versa. In summary, due to the geometry of the system and the nature of the hologram, the inventors identified that there were a plurality of zones (i.e., subareas) of the waveguide from which the two eyes would receive light encoded with different (and, crucially, non-overlapping) parts of the image. One skilled in the art will appreciate that the approach described in Figure 11 approximates a complex optical system, and, for example, the finite size of the entrance pupils may allow for some angular content crosstalk between the eyes. However, the inventors have found this method to be very effective in identifying light-blocking configurations that significantly reduce crosstalk between the two eyes, while still displaying the complete image to both.

In more detail, Figure 11 shows, by way of example, four different zones, where each zone (zone 1, zone 2, zone 3, zone 4) defines a different range of light angles to be emitted from the waveguide towards the observer. Each zone (zone 1, zone 2, zone 3, zone 4) corresponds to a different angular channel and therefore to different image content. In other words, due to the unique way in which the hologram has been calculated, each zone represents a different part of the image and is therefore said to correspond to different image content. The zones are adjacent and contiguous to each other, so that, in combination, they cover the entire range of angles that make up the image and therefore provide all the image content necessary for the observer's brain to holographically reconstruct it upon receiving the spatially modulated light. The reader will appreciate that the number and position of the zones are variable, and Figure 11 shows only one possible example.

The present inventors have recognized that, for each part of the image, and therefore for each region, light should preferably be received by only one entrance pupil (i.e., by one of the observer's eyes, not both), to avoid observer confusion. In particular, the present inventors have devised a control device, at one or more selected locations between the waveguide and the observer, to ensure that light from each part of the image (i.e., each angle) is received by only one of the observer's eyes at any time. The control device may comprise one or more holes, or openings, and one or more barriers or blocks. The control device may be referred to as an "aperture" or "waveguide aperture," having closed and open portions. The waveguide aperture(s) may be configured so that the first observer's eye receives angular content that cannot be received by the second eye, and vice versa. The control device may also be implemented as a light shutter device comprising a plurality of elements forming "shutters" that can be selectively opened or closed.

An example of the waveguide aperture 1100 is shown in Figure 11, in a first phase 1100A and a second phase 1100B. These phases correspond to the "configurations" or "light-blocking configurations" as described above. The present disclosure is not limited to the particular configurations shown in the Figures. These are illustrative examples only.

The waveguide aperture 1100 is shown adjacent to a graph in Figure 11, similar to that of Figure 10, to show how the waveguide aperture 1100 would affect the light that would otherwise propagate towards the observer's eyes, through the waveguide, from zones 1 to 4 of a hologram. Each phase 1100A, 1100B represents a possible configuration that can be applied to ensure that only one eye receives light from each zone at a given time. For each phase 1100A, 1100B, the waveguide aperture 1100 comprises open and closed portions, each of which corresponds to a range (in millimeters, mm) of positions along the waveguide (Pwg) from which light could be emitted. In practice, the waveguide aperture 1100 may be physically located near the waveguide, extending across the light paths between it and the observer, to selectively allow or block certain light paths, as detailed below. For example, the waveguide aperture may be located immediately in front of the waveguide. In this arrangement, the waveguide aperture may be tilted with respect to the plane of the entrance pupil; for example, it may be substantially parallel to the elongated face of the waveguide. However, other locations and orientations of the waveguide aperture are contemplated, which would provide a similar function.

A fixed waveguide aperture may be provided, corresponding to phases 1100A, 1100B, wherein the waveguide aperture is positioned between the waveguide and the observer to allow light emitted from certain positions along the waveguide (Pwg) to reach the observer and block light coming from other positions, as defined by the open (white) and closed (black) sections shown in Figure 11. A plurality of different fixed waveguide apertures may be provided, wherein a selected one of the plurality may be disposed adjacent the waveguide at any given time to provide selective control of spatially modulated light between the waveguide and the observer. Alternatively, a dynamically reconfigurable waveguide aperture may be provided, alternating between the first phase 1100A and the second phase 1100B. By considering each phase, the waveguide aperture 1100 can be understood in more detail as follows:

For the first stage 1100A, the waveguide aperture is spatially divided into 5 portions, each of which defines a range of positions along the waveguide (P<wg>), wherein the portions alternately open and close. In more detail, a first open portion 1121 is defined on the left side of the Figure. It should be appreciated that position terms such as "left", "right", etc., are used solely to facilitate understanding of the examples shown in the Figures and are not to be construed as limiting. The first open portion 1121 defines a range of positions along the waveguide (P<wg>) in which the right eye receives light from region 4. The left eye does not receive light from this range of positions. From left to right, immediately adjacent to the first open portion 1121 is a first closed portion 1122, which defines a range of positions along the waveguide (P<wg>) for which the right eye could receive light from zone 3. However, being a closed portion 1122, it ensures that the right eye does not receive light from zone 3. Furthermore, the first closed portion 1122 also covers a range of positions from which the left eye could have received light from zone 4. However, again, being a closed portion 1122, it ensures that the left eye does not receive light from zone 4. Moving further to the right, immediately adjacent to the first closed portion 1122 is a second open portion 1123, which defines a range of positions along the waveguide (P<wg>) for which the left eye receives light from zone 3. Furthermore, that same range of positions along the waveguide is the range for the which the right eye receives light from zone 2. Moving further to the right, immediately adjacent to the second open portion 1123 is a second closed portion 1124, which defines a range of positions along the waveguide (PwG) for which the left eye would receive light from zone 2. However, being a closed portion 1124, it ensures that the left eye will not receive light from zone 2. Additionally, the second closed portion 1124 also covers a range of positions from which the right eye would receive light from zone 1. However, again, being a closed portion 1124, it ensures that the right eye will not receive light from zone 1. Finally, immediately adjacent to the second closed portion 1124 is a third open portion 1125. It defines a range of positions along the waveguide (P<wg>) for which the left eye receives light from zone 1. The right eye does not receive light from this range of positions. Therefore, when the waveguide aperture is in a configuration represented by the first phase of 1100 A, light from each zone can enter one eye and is prevented from entering the other. This prevents duplication of the received image content.

For the second stage 1100B, the waveguide aperture is spatially divided into 5 parts, which define the same position ranges along the waveguide (PwG) as the corresponding parts of the first stage 1100A. However, in the second stage 1100B, the parts are alternately closed and opened when viewed from left to right, as defined in Figure 11. In detail: a first closed part 1121' prevents light from zone 4 from entering the right eye. It does not affect the left eye, since it does not receive light from this position range. To the right, immediately adjacent to the first closed part 1121', is a first open part 1122'. This allows light from zone 4 to enter the left eye and light from zone 3 to enter the right eye. To the right, immediately adjacent to the first open portion 1122', is a second closed portion 1123'. It prevents light from zone 3 from entering the left eye and light from zone 2 from entering the right eye. Moving again to the right, adjacent to the second closed portion 1123', is a second open section 1124'. This section allows light from zone 2 to enter the left eye and light from zone 1 to enter the right eye. Moving again to the right, adjacent to the second open portion 1124', is a third closed section 1125'. This section prevents light from zone 1 from entering the left eye. Therefore, the functions of the two stages of the waveguide aperture are summarized in Table 1, below.

The embodiments describe an arrangement in which a first and second aperture configuration are opposite, i.e., perfectly complementary, by way of example only. It is not essential that the phases of the control device be perfectly complementary. In other embodiments described herein, more complex configurations and phases are used, especially when the finite size of each entrance pupil is fully considered.

Table 1

The hologram is calculated by dividing the target image content by angle, as described herein with reference to Figures 7A-C. Various methods may be used to calculate such a hologram. In general, the hologram can be said to be constrained according to at least one entrance pupil of the viewing system. The distinctive feature of the hologram is the angular division of the image content into channels. According to embodiments, the hologram is calculated using a point cloud method, as detailed above, which utilizes individual virtual image points from the virtual image to be generated. However, this method is described by way of example only, and other methods may be used to calculate a hologram characterized by the angular channeling of the image content. In embodiments utilizing the non-limiting exemplary point cloud method, each virtual image point may correspond to an angle of the light ray between the display device and an entrance pupil of the viewing system. Each virtual image point may be considered an individual component of the image. In some embodiments, a virtual image region may comprise a single virtual image point or a plurality of virtual image points. In some embodiments, two or more adjacent virtual image points may be comprised within a virtual image region, wherein the angular range of said region comprises all angles corresponding respectively to the individual virtual image points comprised within the region.

In order to successfully control the content reaching each eye (or entrance pupil) of an observer (or viewing system), where each eye (or entrance pupil) occupies a different respective viewing position, each portion of the waveguide aperture described herein (i.e., each zone) defines a maximum light ray angle and a minimum light ray angle receivable from each viewing position, e.g., from each eye when the viewing system is a two-eyed human observer. To prevent overlap of received holographic content between viewing positions (e.g., between the two eyes), for each zone, the maximum light ray angle from the first eye position is less than the minimum light ray angle from the second eye position. If the two viewing positions are configured to receive content from two adjacent zones, the maximum light ray angle from the first eye position is substantially equal to the minimum light ray angle from the second eye position. For example, the waveguide aperture may be configured such that the first viewing position receives spatially modulated light corresponding to the content in an angular range up to and including its maximum light ray angle (>0<max1) and the second viewing position receives spatially modulated light corresponding to the content in an angular range greater than, but not equal to, >0<max1, up to a maximum light ray angle of the second eye position >0<max2.

According to embodiments, the "target image," for which a hologram is calculated and holographically reconstructed according to the present disclosure, is different for each eye. Therefore, the holograms corresponding to the image, calculated separately for each eye, may, in fact, each be a hologram of a different image. In other words, the hologram calculated for the left eye corresponds to the image seen from the perspective of the left eye, and conversely, the hologram for the right eye corresponds to the image seen from the perspective of the right eye.

Thus, the present inventors have recognized that the image content within each region may be different for each eye (or, for each aperture, for any other multi-aperture viewing system). Thus, according to embodiments, the waveguide aperture disclosed herein may be configured to deliver light from all regions to each eye within a narrow time window such that the brain (or processor associated with a non-human viewing system) perceives that each eye has received the entire hologram content of its respective image substantially simultaneously.

According to embodiments, the waveguide aperture may be dynamically configured, and/or there may be more than one aperture, light shutter, or other control device, wherein a suitable trigger controller (as described below, for example) may dynamically control switching between its different configurations, or between the different control devices. Preferably, this should be performed very quickly, for example, faster than the typical integration time of the human eye.

This switching allows the observer to receive all the spatially modulated light emitted by the control device in both phases/configurations in a very short time interval, so that he perceives it to have been received simultaneously.

The waveguide aperture 1100 may be controlled to alternate rapidly between the first and second phases, for example, faster than the typical integration time of the human eye, such that the images received by the observer at each phase are interlaced, as described in GB2108456.1. In this way, the observer perceives that they have seen the entire image with both eyes, each from their own unique perspective based on their position. However, the observer does not perceive the image deterioration or confusion that would be caused by receiving common or overlapping images with both eyes simultaneously and from the same angle. Therefore, the control device works in combination with the calculated hologram(s), as described herein, to create clear and accurate holographic images for the observer. This is achieved in a simple yet effective manner.

The inventors discovered that the embodiments described above can be suitably determined by the center-based geometry of each entrance pupil. In a further improvement described in GB2108456.1, with reference to Figures 26 to 30, the inventors considered the finite size of the entrance pupils. According to these embodiments, image quality is further improved and crosstalk between the different display systems is reduced or even eliminated. One skilled in the art will appreciate how the method described below can be used to dynamically reconfigure the control device and the aperture arrangement in real time, for example, in response to a change in eye or head position, and/or pupil size. The aperture configuration provided by the control device is software reconfigurable, so that the system disclosed herein can also be automatically adjusted in response to vision system parameters such as the separation between the two vision systems (e.g., the interpupillary distance of a human observer).

In some embodiments, a measurement of the pupil diameter is made (e.g., by an eye-tracking system) and this is used as part of the shutter control system. For example, a driver's pupil may vary considerably. In bright ambient light, the pupil diameter will be smaller. An advantage of the system disclosed herein is that, in bright light conditions, when the pupil diameter is relatively small (e.g., 2 mm), less closed aperture area is required to eliminate ocular crosstalk. This is advantageous because it means that the overall efficiency (amount of light propagated toward the observer) is relatively high. Therefore, there is excellent synergy between the shutter system of the present disclosure and the image display, in particular, the head-up display (HUD) in a vehicle.

According to other embodiments, a more complex light shutter scheme may be employed, in which the size of the shutter zones may change during operation of the dynamic shutter. Furthermore, the position of the shutter zones may change or be reconfigured during operation. For example, the position of the boundaries of the shutter zones in an arrangement may change based on the eye position (corresponding to the center of each entrance pupil, upon which the geometry is based), determined by an eye-tracking device. Any number of different shutter schemes may be implemented to ensure that no part of the image is received by multiple viewing positions (e.g., eye positions) simultaneously. This is again achieved by ensuring that each angle of light from the waveguide pupil expander only reaches one viewing position (e.g., one eye) at a time. For example, three different shutter configurations may be implemented in sequence (e.g., cyclically) during operation, as described in the co-pending patent GB2108456.1, with reference to Figures 28 to 30 thereof.

Based on this geometry, the inventors have determined a dynamic shutter scheme that eliminates crosstalk between two eyes having a finite pupil size. In this embodiment, the dynamic shutter scheme comprises three configurations (phases or stages). That is, three different shutter arrangements/patterns are used to reconstruct each hologram. The three shutter arrangements are formed sequentially. Therefore, the different arrangements can be said to be interleaved in time. However, the present disclosure is not limited to three different shutter arrangements, and any number of shutter arrangements can be conceived within the scope of this disclosure.

The control device described herein may take different forms, but, according to the claimed invention, it is a light shutter device. As described below, with reference to Figures 12 and 13, the control device comprises a light shutter device formed by a plurality, as a 2D array, of individually controllable light receiving/processing elements, such as pixels. In particular, the light shutter device may comprise a pixelated liquid crystal device or display. In some embodiments, the elements or pixels may operate in contiguous groups to form the transmissive and non-transmissive shutter regions described herein. Each group of pixels may be switchable between a first optical mode (e.g., transmissive or optically transparent) and a second optical mode (e.g., reflective or optically absorptive/opaque). One skilled in the art knows how to control a pixelated display device so that the size and position of the groups or regions of pixels (each region having a different response to light) can be modified during operation, for example, in real time. Each region is larger than the pixel size of the device. Therefore, each region may comprise a plurality of pixels. One skilled in the art is equally familiar with the implementation of optical components, such as polarizers and wave plates, in conjunction with a pixelated liquid crystal device to provide a reconfigurable light shutter device. By way of example, the control device may utilize polarization selection, but other schemes based on other properties of light are also applicable. In some embodiments, the control device comprises a pixelated liquid crystal display and, optionally, other optical elements collectively configured to transmit light with a first polarization and absorb or reflect light with a second polarization, wherein the first and second polarizations are opposite or complementary. For the avoidance of doubt, any number of different optical systems may be used to form the control device, depending on the characteristics, such as polarization and wavelength, of the image-forming light, and therefore, the present disclosure is not limited by the construction of the control device. Therefore, it will be understood that the control device described herein is defined by its functionality and not by its structure.

The control device is dynamically reconfigurable. In some embodiments, the control device is pixelated. That is, the control device comprises an array of individually controllable pixels. Each pixel may comprise, for example, a liquid crystal configurable between a transmissive and a non-transmissive state. Any imperfection in the alignment between the edge of a pixel and the edge of the ideal aperture zone, identified according to this disclosure, can be resolved by allowing too much or too little light to pass through. That is, by opening or closing another line of pixels in the zonal configuration. In embodiments comprising three or more configurations (for example, when considering the finite size of each entrance pupil), the control device has sufficient resolution or number of pixels such that the pixel at the interface can always block light.

As described above, the control device, such as a light shutter device forming a waveguide aperture, needs to switch quickly between configurations. This is because, during the transition between configurations, the light source illuminating the display device must be turned off to prevent uncontrolled light emission from the waveguide. In embodiments implementing a gated laser light source to illuminate the display device, it is desirable for the shutter device to be fully updated within the time when the laser diode would already be "off" for the laser gate (i.e., during the frame update of the display device). In this way, the allowable on-time of the laser to illuminate the display device is not reduced.

The claimed invention proposes a control device configured as a plurality of pixels, as a one-dimensional array, wherein each pixel forms a "shutter" for selectively blocking light from a corresponding transmission point along the waveguide. Therefore, the control device is referred to as a "light shutter device." The pixels are liquid crystal cells, operable in two optical states. In one optical state, the liquid crystal cell blocks (e.g., absorbs or reflects) incident light from a corresponding position (or region) in the waveguide, to prevent light transmission. In the other optical state, the liquid crystal cell is transparent, such that incident light from a corresponding position (or region) in the waveguide is transmitted therethrough. The pixels of the light shutter device are controlled by control signals, as is well known in the art. In some embodiments, all pixels of the light shutter device are directly driven by one of two drive signals to drive the pixel into one of the first and second optical states.

Figure 12 shows a trigger scheme for switching the configuration of a light shutter device 1200, which comprises a one-dimensional array of liquid crystal cells forming pixels. The trigger circuit comprises a plurality of switches 1210, each associated with an individual pixel of the light shutter device. It should be noted that, in other arrangements, each switch may be associated with an independently controllable group of pixels. Each switch may be selectively connected to a first or second trigger signal. In the illustrated arrangement, the first trigger signal is a "low" signal, where the switch is connected to the low voltage line or ground to activate the corresponding pixel(s) in a transparent state (illustrated as white/unshaded) to open the shutter. The second trigger signal is a "high" signal, in which the switch connects to the high-voltage line to activate the corresponding pixel(s) in an opaque or reflective state (illustrated as black/shaded) to close the shutter. Other arrangements are possible and contemplated.

Accordingly, in the gating scheme of Figure 12, each pixel of the light shutter device 1200 is directly driven by a "low" or "high" drive signal under the control of a corresponding switch 1210. Direct gating of the pixels maximizes the contrast between the transparent and opaque states, as compared to a passive gating scheme. Furthermore, independent gating of each pixel maximizes the flexibility in defining the groups of pixels that form the different "zones" of the light shutter device, as described herein. For example, the boundary between zones may need to be changed in response to changes in the eye position of the observer's eyes within the eye box, corresponding to the center of each entrance pupil on which the geometry is based, as described above. The position of the observer's eyes may be tracked by an eye-tracking device, as is well known in the art.

In some embodiments, the switches are arranged externally to the light-shuttering device, such as on an external printed circuit board (PCB). In other embodiments, the switches may be implemented as part of the pixel circuitry associated with each of the pixels in the light-shuttering device. For example, a switch may be implemented within each pixel circuitry integrated into the substrate of a thin-film transistor liquid crystal display (TFT-LCD) that forms the light-shuttering device.

However, for liquid crystal cells according to the claimed invention, such as pixels in a thin-cell liquid crystal device, the switching time period to transition from one optical state to another is longer than that to transition from one to the other. For example, the switching time period is longer when the liquid crystal cell relaxes to its new state than when it is activated to that state. The time period to transition from a second optical state (shutter closed) to a first optical state (shutter open) is longer than the time to transition from the first optical state (shutter open) to the second optical state (shutter closed), such as when the liquid crystal cell relaxes to the transparent/open state but is activated to the opaque/closed state.

The present disclosure addresses the technical problem that the switching time period of the pixels of a light shutter device, when they change in a direction whereby the liquid crystal cell "relaxes" to its new state, is too long for the application requirements, such that performance is compromised as described above.

Figure 13 shows a system for controlling a light shutter device 1300, according to embodiments, allowing its settings to be updated more quickly and efficiently. The light shutter device 1300 comprises a one-dimensional array of pixels, as described above with reference to Figure 12.

The system, referred to as a "drive circuit," comprises a drive controller 1330, a multiplexing circuit 1320, and a switching circuit 1310. The switching circuit 1310 comprises a plurality of switches, such as digital switches. In the illustrated arrangement, the number of switches in the switching circuit is equal to the number of pixels in the light shutter device 1300. The output of each switch in the switching circuit 1310 is connected to a corresponding pixel in the light shutter device 1300, to independently drive the pixel with a "low" or "high" drive signal.

The trigger controller 1330 is arranged to control the switching of the pixels of the light shutter device 1300 by the switching circuit 1310, thereby changing their configuration during an update cycle. The controller 1330 sends control signals to the switching circuit 1310 to control the selection of the updated trigger signal (high or low) to be provided to each pixel, as well as the timing, sequence, and update order of each pixel of the light shutter device 1300. In the illustrated arrangement, a multiplexing circuit 1320, comprising one or more stages of multiplexers, is used to sequentially route control signals from the controller 1330 to the plurality of switches of the switching circuit 1310. It will be appreciated by one of skill in the art that other arrangements, in place of the multiplexing circuit 1320, may be used for signal routing. The trigger controller 1330 is typically arranged to receive inputs from the display system and generate the control signals necessary to reconfigure the light shutter device 1300 at each refresh cycle at an appropriate time (e.g., during a frame refresh of the display device). For example, the trigger controller 1330 may receive signals from a holographic engine or a display driver of the display device, indicating the refresh of a frame or subframe of the device. Additionally, or alternatively, the trigger controller 1330 may receive signals from a light source controller indicating when the light source is turned on and/or off. The trigger controller 1330 may receive signals from an external processor indicating updated configuration or configuration changes needed for the light shutter device 1300. Alternatively, the trigger controller 1330 itself may determine the necessary configuration or configuration changes. In the latter case, the trigger controller 1330 may receive signals from an eye-tracking system indicating changes in the position of the observer's eyes to determine the position of the zones, as described above. The drive controller 1330 may be implemented as an external processor or circuit, such as an FPGA.

To ensure that the total time period for switching all of the pixels of the light shutter device 1300 during one refresh cycle is within the required time, e.g., less than the time during which the light source is turned off during a frame refresh of the display device, the trigger controller 1300 is arranged to provide the control signals to the plurality of switches in an optimal sequence. In particular, the trigger controller 1330 is arranged to determine an optimized sequence defining the order in which the control signals are provided to the plurality of switches based on the change in state of the corresponding pixels by updating the configuration. In accordance with the claimed invention, the trigger controller 1330 identifies pixels that are to be driven in a direction that takes longer to fully switch between states (e.g., the direction in which the pixel "relaxes" to the new state) and determines the sequence of control signals so that the identified pixels are switched first, before any others. Typically, the sequence of control signals for the switches that are associated with the identified pixels (e.g., zones) to be switched first is such that the trigger signals trigger consecutive pixels in the array of the light shutter device 1300. In this way, the identified pixels (zones) are triggered sequentially (i.e., one by one) from a first end of the array to a second end of the array. After all of the identified pixels have been triggered, the remaining pixels (zones) are triggered sequentially (i.e., one by one) from the first end of the array to the second end of the array. In this way, all pixels that take the longest to reach a new stable state will be fully triggered by the end of the update cycle.

In some embodiments, each of the plurality of switches in the switching circuit 1310 may comprise a digital switch with inputs connected to first and second voltages corresponding to the high and low trigger signals. A control input of each digital switch controls the selection between the high and low control signals output from the switch. Accordingly, in these embodiments, control signals are sequentially received as control inputs to the digital switches to control the output of the high or low signal required to sequentially trigger their respective pixels of the light shutter device. In this way, each digital switch is addressed one by one, in sequence. However, since the total update time (the time taken for all pixels in the light shutter device to update their configuration) is limited, as mentioned above, the control signal is applied to each switch for a very short time, after which a floating voltage may exist at the control input. In particular, the inventors discovered that a digital switch might not be able to reliably maintain its state, in order to output the correct drive voltage to its respective pixel, when the control signal is applied for such a short time. To overcome this problem, the inventors propose to provide a "sample and hold" capacitor in the signal path between the drive controller 1330/multiplexing circuit 1320 and the control input of each digital switch in the switching circuit 1310. The sample and hold capacitor maintains the applied control signal voltage received from the drive controller 1330/multiplexing circuit 1320 for a longer period of time, such that the digital switch operates reliably.

The examples described above should not be considered limiting. For example, the vision system may have more than two viewing apertures or entrance pupils. For example, it is possible to divide an image (and, consequently, a calculated hologram and the resulting holographically reconstructed image) into any number of zones, and thus produce any number of corresponding angular channels of spatially modulated light. For example, the control device has been described as being switchable between first and second stages, but could be configured to switch between more than two stages. For example, the control may have four stages. According to one embodiment, one stage (e.g., each stage) of the control device may allow light to be delivered to only one eye (or viewing aperture). According to one embodiment, one stage (e.g., each stage) of the control device may allow light to be delivered to only more than one eye (or more than one viewing aperture).

The number and/or size of the areas receiving image content need not necessarily be equal in each phase. In other words, some phases of the control device may provide more image content than others. Similarly, both eyes need not receive the same amount of image content in each phase, or in total, when multiple phases of the control device are interlaced. For example, one eye may see more image content than the other, depending on their relative positions and/or other factors.

A display system comprising a control device, such as a waveguide aperture, as described herein, may be configured to display a plurality of different images, one after the other and/or at different times. Thus, a display device within such a system may be configured to display different holograms, sometimes in rapid succession. The different images may have a different number of zones. Furthermore, the size of the zones may differ between respective images. Similarly, light rays from the zones of one image may be defined by different angles with respect to light rays from the zones of a second image. The control device may be configured to dynamically adapt to changes in the number and/or size of the zones.

In other words, the precise positions along the waveguide (Pwg) at which the control device transitions from open to closed, and vice versa, may not be fixed. Instead, the control device may be configured to dynamically change the positions along the waveguide (Pwg) at which it transitions from open to closed. Thus, the total number of open and closed portions provided by the control device may be varied. The individual sizes of one or more of these portions may also be varied.

The system may be configured to display a sequence of images, such as a video-rate image sequence. Each image may correspond to a frame in a sequence of frames with a frame rate of, for example, 50 or 60 Hz. Each frame may comprise a plurality of subframes. The rate of the subframes may be, for example, 4 or 8 times faster. The displayed hologram may change for each successive subframe. Each subframe may be considered an individual viewing event. Each subframe may correspond to the image or at least a portion of it. While in some embodiments it has been shown that light is supplied to both eyes at each viewing event, the present disclosure is not limited in this regard. For example, the light engine may be configured to supply light to only one eye/input pupil per viewing event. The waveguide aperture configuration (i.e., the size and/or distribution of the open and closed apertures/holes) may change on each viewing event or every n viewing events, where n is an integer. Furthermore, while the described embodiments show that adjacent image content is delivered through each aperture/hole during a viewing event, the present disclosure is not limited to such a situation, and the image content delivered through each aperture may not be adjacent. In some embodiments, only one angular range of light is delivered to an eye per aperture configuration/viewing event. In some embodiments, the control system is configured to deliver light to each input eye/pupil, one at a time.

As described above in the present disclosure, a hologram of a target image may be calculated for a specific size and position of a viewing aperture, for example, for a specific size and position of the entrance pupil of the observer's eye. If a constraint, such as the diameter or position of the entrance pupil, changes, the hologram may be recalculated, even if the target image to be reconstructed at that time (and thus the image content that the observer will see or perceive) remains unchanged. Each hologram need not have the same number or size of regions, even when two holograms represent the same target image.

The control device, such as a light-shutter device, has been described specifically in the context of channeling hologram propagation. However, it should be clear that the advantages of the control device (particularly the provision of a light-shutter device with a fast update time that compensates for slower state changes depending on the switching order) are also applicable in other contexts.

Additional features

Embodiments relate to an electrically driven LCOS spatial light modulator by way of example only. The teachings of the present disclosure may equally be implemented in any spatial light modulator capable of displaying a computer-generated hologram in accordance with the present disclosure, such as any electrically driven SLM, optically driven SLM, digital micromirror device, or microelectromechanical device, for example.

In some embodiments, the light source is a laser, such as a laser diode.

The system of the present disclosure may be used to provide an enhanced head-up display (HUD) or head-mounted display. In some embodiments, a vehicle is provided comprising the holographic projection system installed to provide a HUD. The vehicle may be an automobile, such as a car, truck, van, motorcycle, train, airplane, boat, or watercraft.

The examples describe illuminating the SLM with visible light, but the skilled person will understand that the light sources and the SLM can also be used to direct infrared or ultraviolet light, for example, as described herein. For example, the skilled person will understand techniques for converting infrared and ultraviolet light into visible light to provide information to the user. For example, the present disclosure covers the use of phosphors and/or quantum dot technology for this purpose.

The methods and processes described herein can be implemented on a computer-readable medium. The term "computer-readable medium" includes a medium arranged to store data temporarily or permanently, such as random access memory (RAM), read-only memory (ROM), buffer memory, flash memory, and cache memory. The term "computer-readable medium" also includes any medium, or combination of media, capable of storing instructions for execution by a machine, such that these instructions, when executed by one or more processors, cause the machine to execute one or more of the methodologies described herein, in whole or in part.

The term "computer-readable medium" also encompasses cloud storage systems. The term "computer-readable medium" includes, but is not limited to, one or more tangible, non-transitory data repositories (e.g., data volumes) in the form of a solid-state memory chip, optical disk, magnetic disk, or any suitable combination thereof. In some embodiments, execution instructions may be communicated via a carrier medium. Examples of such a carrier medium include a transient medium (e.g., a signal that propagates and communicates instructions).

It will be apparent to those skilled in the art that various modifications and variations may be made without departing from the scope of the appended claims. This disclosure encompasses all modifications and variations within the scope of the appended claims and their equivalents.

Claims (15)

1. A light engine arranged to form an image visible from a viewing window, wherein the light engine comprises: a display device arranged to display a hologram of the image and spatially modulate light in accordance with the hologram; a waveguide pupil expander (902) arranged to receive the spatially modulated light and provide a plurality of different light propagation paths for the spatially modulated light from the display device to the viewing window; and a light shutter device (1200) arranged between the waveguide and the viewing window, the light shutter device comprising: a plurality of liquid crystal cells, wherein each liquid crystal cell is operable in a first optical state or a second optical state in response to a respective first or second drive signal; a drive circuit comprising: a plurality of switches (1210), each switch being arranged to input a respective first or second drive signal to a respective liquid crystal cell; and a drive controller arranged to sequentially update the output of each switch during a refresh cycle; wherein the trigger circuit is arranged to determine the order in which the switches are sequentially updated during an update cycle based on any changes in the respective trigger signals that will occur during the update; wherein the liquid crystal cells take longer to change from the second optical state to the first optical state than from the first optical state to the second optical state, and the driving circuit is arranged during a refresh cycle to update the output of any switch to be changed from the second driving signal to the first driving signal before any other switch. 2. A light engine according to any preceding claim, wherein the trigger circuit is arranged during the update cycle to update the output of any switch that will be switched from the first trigger signal to the second trigger signal after all switches that will be switched from the second trigger signal to the first trigger signal have been updated. 3. A light engine according to any preceding claim, wherein the plurality of switches (1210) comprises a plurality of digital switches, each digital switch comprising a first input arranged to receive the first drive signal and a second input arranged to receive the second drive signal; and optionally further comprising a capacitor associated with each digital switch, each capacitor being arranged to receive a drive controller signal during a refresh cycle and to provide a control input to the respective digital switch for selectively outputting either the first or the second drive signal to the respective liquid crystal cell, the capacitor being arranged to maintain the drive input to the digital switch between refreshes. 4. A light engine according to any preceding claim, wherein the driving circuit further comprises a multiplexing circuit for sequentially routing control signals from the driving controller to update the output of each of the switches during a refresh cycle; and/or wherein the liquid crystal cells are arranged in a one-dimensional array. 5. A light engine according to any preceding claim arranged to restrict the area of the output face of an optical replicator that is visible from a viewing plane, optionally, wherein the optical replicator is arranged to output a plurality of replicas of a hologram. 6. A light engine according to any preceding claim, wherein the light shutter device (1200) is disposed between a display device and an observer; optionally, wherein at least one update cycle is completed within the integration time of the human eye; optionally, wherein an update cycle is initiated if a change in the viewing position of the observer is detected. 7. A light engine according to any preceding claim, wherein the light shutter device (1200) is coupled to an exit face of the waveguide pupil expander (902); and/or wherein the light shutter device (1200) restricts the area of the exit face of the waveguide visible from the viewing window. 8. A light engine as claimed in any preceding claim, wherein the waveguide pupil expander (902) and the viewing window are not parallel. 9. A light engine according to any preceding claim, wherein the hologram is configured to angularly distribute the spatially modulated light of the image according to the position of the image content, such that the angular channels of the spatially modulated light correspond to respective continuous regions of the image; and wherein at least one liquid crystal cell of the plurality of liquid crystal cells of the light shutter device forms an opening arranged such that a first viewing position within the viewing window receives a first channel of light spatially modulated by the hologram according to a first region of the image, and a second viewing position within the viewing window receives a second channel of light spatially modulated by the hologram according to a second region of the image. 10. A light engine according to claim 9, wherein the light shutter device (1200) comprises at least one hole; each hole provides the first and/or second viewing position with a respective channel of spatially modulated light, such that different image content is delivered, respectively, to the first and second viewing positions at substantially the same time. 11. A light engine according to claim 10, wherein the light shutter device (1200) is configured such that each hole is switchable between an open position and a closed position, so that a plurality of different configurations of the light shutter device are provided, wherein each configuration comprises an alternating sequence of open and closed holes. 12. A light engine according to claim 10 or 11, wherein the light shutter device (1200) is configured to provide a first control device configuration at a first time and a second control device configuration at a second time, wherein the first control device configuration and the second control device configuration are complementary. 13. A light engine as claimed in claim 12, wherein the time interval between the first moment and the second moment is less than the integration time of the human eye. 14. A light engine according to any one of claims 9 to 13, wherein a first control device configuration, provided by the light shutter device (1200), delivers modulated light according to the first and third image zones to the first viewing position, and modulated light according to the second and fourth image zones to the second viewing position, wherein the first to fourth zones are contiguous and ordered areas of the image; optionally, a second control device configuration, provided by the light shutter device, provides (1200) modulated light according to the second and fourth image zones of the image to the first viewing position, and modulated light according to the first and third image zones of the image to the second viewing position. 15. A light engine according to any preceding claim, wherein the first and second viewing positions are the ocular positions of an observer, and the viewing window is an eye-box; and wherein the size and/or location, within the light-shuttering device (1200), of at least one of the holes is dynamically variable.